Diy Ups

[MORA99]

Hello again, I am looking at building a UPS to keep the network and a server running during "short" power failures, and ofcause safe shutdown, alarms, temperature monitoring, all the stuff that you dont get from a black box marked "UPS"

My last UPS died out after 3 years service so I have been thinking about making my own.

Since most of the equipment runs on some DC voltage, I should be able to get down to < 50W on the inverter (which I would be running all the time to prevent delay in switching).
Then the rest will run off 12V and a few step up/down switchers for the stuff that cant take 12-14V directly.

I have located a 70Ah AGM battery, and will ofcause try to build the charger my self, using pulse width and a FET (I have read the old threads on the subject).
I can get a 13.8V dc generator (although I might need 2 to handle the load) which will power the charge circuit and the equipment.
(picaxe circuit ofcause on a 5V regulator)

The question is not so much the battery charge circuit, but the backup system.
The easy way is to connect the load "directly" to the battery and the charge system ofcause to the battery also.
However this means the charger has to charge at C/20 + what ever is being used by the system, which is kinda hard to know if the battery is flat.

An alternative would be to use a mechanical relay.
Common ground.
NO connected to the mains adapter.
NC connected to the battery.
Coil connected to the mains adapter.

While mains is active, the dc generator will power the coil and let the equipment draw from the generator directly.
When mails fail the coil is not powered, and NC is connected to the load.
However would this switch be fast enough for all the equipment to survive, im guessing no since the voltage will drop down to maybe 7V before the coil even drops, and this is the same voltage the equipment is seeing ?
I have not looked into which switchers to use for the DC so I dont know if they will accept a 5-15V range, and keep same output.

It would make the charge circuit somewhat easyier since it just have to keep the battery topped (I expect 5 dischages to 80% a year, and maybe 1 "full" discharge every 2years, battery would need replacement in 2-4years since its not top quality name).


The power for the mains regulator will be 230Vac, and the total DC load will be around 150W (So 20A to allow for charge+load).

Any ideas/surgestions ?
Is 15A (C/4.67) too high discharge current for a standard AGM (this quality does not come with datasheets, and is typically rebranded).

 

[hippy]

With genuine APC and other UPS's of 1200VA at sub-£40 ( including batteries ) it may be a false economy to try and build one's own although it's true you can add all sorts of gizmo's to your own design.

On the DC side of things you have a point; it seems silly to bump everything up to mains then have a plethora of walwarts all of which seem to run incredibly hot. I'm looking at what I have by way of routers etc and am sifting the 5V and 12V out and am planning to run on a single fanless PC supply.

I must admit it's not clear to me what you have as output ( DC only ? for a server ? ). My UPS isn't continuously regenerative and has a relay to switch from mains to generated AC and that doesn't cause any problems, but I suspect that relies on all equipment having caps which will ride them through the few milliseconds or so switch over.

 

[demonicpicaxeguy]

have you investigated what actaully died?, was it the battery inside or somthing on the board?

it's worth looking at, eg my father who works as a subcontractor to some of the big banks has brought home ups's in the past which i've pulled apart and found things like batteries to simply be cactus and need replacing, one of them just needed a fuse replaced, they are pretty simple devices and you'll probably find it'll be easier to repair the ups and build a new one,

 

[MORA99]

OK, I´ll bite

I have.
Modem (This is really the only 230V unit I cant power from dc, since it has internal adapter and the casing is off limits (fibre optics)).
Router & VoIP (Supplied by the ISP)
2xSwitch
Wlan router
1xDIY network enabled relay box (Picaxe 28X1 with Simplelan and ebay relay card)
And the server, right now its using a regular 230V supply, but since its power usage is low, it could do with a picopsu (which is a small psu that plug directly into the ATX connector and just takes a regulated 12V (or the other versions that takes 14-35V))

The only sub 40GBP here (from APC) is a 350VA which is rated for 6.6minutes.
Hardware is not very cheap in Denmark.
(And it has to be APC, since the reason I have to scrap the entire UPS this time, is that replacement batteries is nowhere to be found (except overseas)).

 

[MORA99]

have you investigated what actaully died?, was it the battery inside or somthing on the board?

it's worth looking at, eg my father who works as a subcontractor to some of the big banks has brought home ups's in the past which i've pulled apart and found things like batteries to simply be cactus and need replacing, one of them just needed a fuse replaced, they are pretty simple devices and you'll probably find it'll be easier to repair the ups and build a new one,

I believe its the battery that is broken, the UPS is still connected, but when the power fails, it instantly (or well 10seconds) sounds the overload alarm and shuts off.

Upon power return it will start alarm again because surge current is over the rated limit, a power button toggle and eveything is working again.

The old UPS is a Trust 1000VA (And it did give fine service for 2years for 55GBP, so its a ok product for the price)

 

[BCJKiwi]

Don't understand the comment;
"And it has to be APC".
APC don't manufacture Batteries (AFAIK) and any good quality Battery of the correct type/size should work fine. Have replaced many UPS batteries over the years with 'standard' batteries.

 

[demonicpicaxeguy]

i'd pull it apart and replace the battery,

 

[MORA99]

Ok, I will have a look at the next "service window" where its being removed (2weeks from now).

I collected some part prices, and my diy would end up in the more expensive ones, but still, this would give me maybe 2 hours of runtime, where many of the commercial consumer grade will give 15minutes.

Part list.
Battery (70Ah AGM "noname") 695,-
300W inwerter 295,- (May need to be another one, does not say if its the good sine type)
20A 13.8DC adapter 400,-
Picaxe circuit and prototypes 200,-

Total 1590DKK
1590 Danish kroner = 186.359817 British pounds

 

[BCJKiwi]

If extended run time (off mains) is required there are typically two options;
1. buy a larger capacity UPS
2. buy a UPS that will accept additional batteries.

There are three types of UPS in common use;
Full time
line interactive
switched.

Full time is mains -> charger -> battery -> inverter -> load (one of the options you described above)
Line interactive is the same plus a bypass of mains to load with switching between the two that is fast enough AND keeps AC wave form synchronized.
Switched - just a relay or fast switch switching of load between mains and inverter.

Line interactive is the most common and is fine for computer systems provided it really works properly as described above. Anything less is unacceptable for computers.

 

[Dippy]

I agree, don't waste your time on something that is relatively cheap - especially where design/safety is a significant issue.

Get some decent batteries.

I know this won't prevent the bee-line to Ebay crappo-cheapo-battery-Company, but heed this:-

I have 4 UPS of different makes.
I have had them for about 8 years.
When the first batteries died I penny-pinched and bought cheap made in Ch batteries that claimed to be suitable for UPS. (Camden).
They lasted about 8 to 12 months depending on UPS.
After that I paid the extra and used Yuasa. They cost 30% more but lasted about 18 months to 2 years.
I don't penny-pinch any more.

AND, don't be tempted to stick a chuffing great battery in a small UPS.

 

[Taniwha]

Some thoughts on the topic of UPS and batteries,
If you let me know the battery details I will see what i can find through our battery manufacturers.

If the UPS is three years old then it is most likely only the batteries that need replacing.
As to the "and ofcause safe shutdown, alarms, temperature monitoring, all the stuff that you dont get from a black box marked "UPS"" Even our most basic UPS offers most of that through serial connection to a P.C., this includes shutdown and restart software, battery monitoring/testing etc.
BCJKiwi is absolutely correct about how to extend runtime however simply buying a larger "entry level" UPS may not help with extending the run time for one of two reasons:
1) Some UPS's will automatically switch off after 15 minutes, regardless of available battery capacity. this is done to prevent the inverter from over heating, and is normally load dependant (i.e. at a predeterimed load the ups will count down then stop)
2) Some UPS's will alos shut down if the load is too small, the UPS sees it as no load and shuts down to protect the batteries, this is often the case when your server is off and there is just the router/switch running.

However if you are serious about building your own online double conversion UPS then good luck, I will post a seperate post shortly outlining a few things you might want to take into consideration.

 

[Taniwha]

I agree, don't waste your time on something that is relatively cheap - especially where design/safety is a significant issue.

Get some decent batteries.

I know this won't prevent the bee-line to Ebay crappo-cheapo-battery-Company, but heed this:-

I have 4 UPS of different makes.
I have had them for about 8 years.
When the first batteries died I penny-pinched and bought cheap made in Ch batteries that claimed to be suitable for UPS. (Camden).
They lasted about 8 to 12 months depending on UPS.
After that I paid the extra and used Yuasa. They cost 30% more but lasted about 18 months to 2 years.
I don't penny-pinch any more.

AND, don't be tempted to stick a chuffing great battery in a small UPS.

Sorry Dippy I have to chuckle at that, we sell a direct equivalent battery to the Yuasa and they are still entry level batteries, however UPS's, particulary Double Conversion (On Line) UPS, are very tough on batteries.
The two biggest killers of AGM type batteries are heat, particulary during the charge period, a good rule of thumb is for every 5 degrees c above 20 degrees c you reduce the battery life by 50%.
The other killer is charge voltage control, AGM batteries are VERY susceptible to overvoltage and A.C. ripple, both of which cause overheating and can lead to a condition called thermal runaway, at best the battery fails, bulging or even melting, at worst there is a very real risk of the battery "expiring in a non passive manner" I.E. catching fire or even exploding.

 

[Dippy]

Well, I chuckle too. Many of us have experienced the crippling effects of unsuitable charge control in many projects Many people on this Forum are actually experienced designers with years of experience in actual nitty-gritty design.


What I'm trying to say is that when "Suitable for UPS" is written on the 'tin' it doesn't mean it's ideal for that application.
And the temptation is usually "Get the Cheapest. I've never heard of that make, but hey, I've saved a quid".


Manufacturers and retailers can make all sorts of claims. If/when the battery fails prematurely the retailer can/will always conveniently blame the customer's UPS. "OH, it's your charge control Sir" or "I think your old socks blocked the vents and it overheated". Never "This battery is cheap chod".
Sorry, I don't mean to sound cynical but I've heard it all before a hundred times.

Funny how Yuasa lasted twice as long under the same conditions isn't it.... (.. Twilight Zone theme tune).

 

[Taniwha]

Hello again, I am looking at building a UPS to keep the network and a server running during "short" power failures, and ofcause safe shutdown, alarms, temperature monitoring, all the stuff that you dont get from a black box marked "UPS"

These alarms and shutdown scripts are standard on every UPS I sell (except internal temperature on some models) I would imagine a lot of coding will go into the picaxe and P.C. to duplicate it. I hope you post your code at the end of the project.

I have located a 70Ah AGM battery, and will ofcause try to build the charger my self, using pulse width and a FET (I have read the old threads on the subject).
I can get a 13.8V dc generator (although I might need 2 to handle the load) which will power the charge circuit and the equipment.
(picaxe circuit ofcause on a 5V regulator)

Be aware that AGM batteries require very tightly controlled charge curves to prevent damage, they also require very low A.C. ripple. For instance our 12 year design life AGm battery has the following specifications: Max charge voltage is 2.4vpc, max allowable ripple is 3.5% charge voltage, float voltage is 2.27 - 2.3 vpc @ 20 deg C.

Also it would pay to check that the battery you are proposing is a "Standby" battery not a "Deep Cycle" or "Start" battery.

The question is not so much the battery charge circuit, but the backup system.
The easy way is to connect the load "directly" to the battery and the charge system ofcause to the battery also.
However this means the charger has to charge at C/20 + what ever is being used by the system, which is kinda hard to know if the battery is flat.

This is a typical application in our industry where the rectifer maintains a standing load AND recharges the battery, it is worth checking what the maximum voltage your equipment can handle as you will need to float the battery at around 13.8 volts.

Most AGM's are designed for Constant Voltage charging.

While mains is active, the dc generator will power the coil and let the equipment draw from the generator directly.
When mails fail the coil is not powered, and NC is connected to the load.
However would this switch be fast enough for all the equipment to survive, im guessing no since the voltage will drop down to maybe 7V before the coil even drops, and this is the same voltage the equipment is seeing ?
I have not looked into which switchers to use for the DC so I dont know if they will accept a 5-15V range, and keep same output.

A bigger concern should be what is the minimum voltage D.C. that your equipment and inverter will run on, and does the inverter have a low voltage cut out. Most inverters have a low D.C. cutout to protect the battery supplying them, this will typically be around 10 - 10.5 volts. We would normally specify a voltage window that is acceptable to the equipment and shut down outside of that window.

It would make the charge circuit somewhat easyier since it just have to keep the battery topped (I expect 5 dischages to 80% a year, and maybe 1 "full" discharge every 2years, battery would need replacement in 2-4years since its not top quality name).

how did you reach these figures? if you are building a true online UPS then EVERY brownout/dip/surge may switch the load to battery momentarily. this is the reason online UPS batteries fail before Line Interactive UPS batteries.

Any ideas/surgestions ?
Is 15A (C/4.67) too high discharge current for a standard AGM (this quality does not come with datasheets, and is typically rebranded).

A typical 70ah battery should happily run 15a for ABOUT 3 hours, to an end voltage of 1.7 volts per cell (vpc) this will give you an end voltage of 10.2 volts from a nominal 12 volt block.
How are you planning to switch off your system once the battery is discharged?

 

[Taniwha]

Well, I chuckle too. Many of us have experienced the crippling effects of unsuitable charge control in many projects Many people on this Forum are actually experienced designers with years of experience in actual nitty-gritty design.


What I'm trying to say is that when "Suitable for UPS" is written on the 'tin' it doesn't mean it's ideal for that application.
And the temptation is usually "Get the Cheapest. I've never heard of that make, but hey, I've saved a quid".


Manufacturers and retailers can make all sorts of claims. If/when the battery fails prematurely the retailer can/will always conveniently blame the customer's UPS. "OH, it's your charge control Sir" or "I think your old socks blocked the vents and it overheated". Never "This battery is cheap chod".
Sorry, I don't mean to sound cynical but I've heard it all before a hundred times.

Funny how Yuasa lasted twice as long under the same conditions isn't it.... (.. Twilight Zone theme tune).

Funnily enough we offer a standard Yuasa comparible battery and a German built three times the price option, but you know what, i would sell several hundred cheap ones to each quality one & when they fail after three to four years and you tell the customer it was a cheap battery, they never accept they should buy the top quality one.

But i entirely agree, just because a battery says "suitable for UPS' doesn't mean its the best option for a UPS.
And no its not surprising that the Yuasa lasted twice as long as "some cheap chod"
The point I am making is that UPS's are hard on batteries, online UPS's are the worst and ALL Sealed Lead Acid batteries are sensitive to temperature, particularly during the charge phase, any designer who doesn't accept that as part of their design brief is looking for trouble.

One last thought, assuming the UPS's are the (typically) sub 6kVA Line interactive or Online models and running well I would expect our Yuasa equivalent battery to last 3 - 4 years in average use in New Zealand.

 

[manuka]

I guess my irregular comment re FREE SLA batteries may be in order. To avoid the prospect of just the sort of 3 year type failures mentioned above, critical UPS & security sites often replace the battery every 2 years. These SLAs (mostly Yuasa & typically 12V 7Ah but often much larger) are usually still in superb condition, & are quite acceptable for non critical use. I've some 5-8 yo. discards here still storing (mostly PV charging) & pumping out serious energy.

Simply phoning the service depot, & turning up at happy hour with a few amber fluid will usually do the trick. At times here in Wellington (NZ) I've has so many SLAs loaded into my car trunk that the front wheels threaten to wave in the air. Stan

 

[Taniwha]

I guess my irregular comment re FREE SLA batteries may be in order. To avoid the prospect of just the sort of 3 year type failures mentioned above, critical UPS & security sites often replace the battery every 2 years. These SLAs (mostly Yuasa & typically 12V 7Ah but often much larger) are usually still in superb condition, & are quite acceptable for non critical use. I've some 5-8 yo. discards here still storing (mostly PV charging) & pumping out serious energy.

Simply phoning the service depot, & turning up at happy hour with a few amber fluid will usually do the trick. At times here in Wellington (NZ) I've has so many SLAs loaded into my car trunk that the front wheels threaten to wave in the air. Stan

ahh so that is where our scrap metal based xmas is going!!!!!

 

[manuka]

Scrap lead prices of course rise & fall, but the 2007-8 price hikes look now history. It's presently ~US$1.50 a kg, but messy batteries fetch only a fraction of that -many firms again can't be bothered. There's more beer money & less hassle in scrap copper of course.

FWIW the nature of the load may well be VERY worth considering. Tales abound of UPSs being used to even boil up a kettle! Given modern energy efficient electronic devices, power hog hardware from even 5 years back may not be worth UPS supporting. It doesn't take too much of a business case to show the all up merits of moving to greener hardware. This is not limited to PCs/servers of course-I've an architect mate (once a halogen fanatic) now considering using LED lighting entirely on a new job, as even CFLs are "so 2004". Stan.

 

[Taniwha]

Scrap lead prices of course rise & fall, but the 2007-8 price hikes look now history. It's presently ~US$1.50 a kg, but messy batteries fetch only a fraction of that -many firms again can't be bothered. There's more beer money & less hassle in scrap copper of course.

FWIW the nature of the load may well be VERY worth considering. Tales abound of UPSs being used to even boil up a kettle! Given modern energy efficient electronic devices, power hog hardware from even 5 years back may not be worth UPS supporting. It doesn't take too much of a business case to show the all up merits of moving to greener hardware. This is not limited to PCs/servers of course-I've an architect mate (once a halogen fanatic) now considering using LED lighting entirely on a new job, as even CFLs are "so 2004". Stan.

The worst "deliberate" load (we all have tales of cleaners vacuum cleaners or receptionist's fan heaters) is Laser printers, the inrush and preheat on those babies can be a killer.
The Lead market is so volatile we need to calculate lead price on the day when we quote batteries for special projects.
Stewart

 

[moxhamj]

I have strange visions of Stan trying to drive home with the front wheels up in the air. Paint some flames on the side of the car and you can have a hotrod.

Re I am looking at building a UPS to keep the network and a server running during "short" power failures,

My shed computer is on a UPS and is stripped down to the bare minimum. The monitor is turned off pretty much all the time. I've got a program 'rain' running on the CPU sending NOP instructions when it is quiet so the CPU runs only mildly warm. No graphics cards. I measured the current at 3A (from the 12V). I think the router might add another 500mA at 12V. At these sorts of currents, a 7aH SLA in a standard UPS works fine and has saved the system needing a reboot for over two years now.

I've pondered many times whether a homebrew UPS would be better than the commercial ones, but you seem to get so much in a commercial unit. The fast switchover. The proper charging/float. The warning beeps, the display with the charge remaining, and even the RS232 connection back to the PC to shut it down gracefully as the battery expires.

 

[MORA99]

wow, lots of messenges since last night (GMT+2).

As some says the UPS ofcause has a RS232 cable, I never really used it due to poor software, but will try again, it could show voltage, temperature and do a 10second battery test.

The estimated power outages I mentioned it mostly when power is cut for some reason, its very cool to be able to cut the power for half an hour and everything is still running (Replace outlets, change fuses, test the HPFI, etc).

Very rarely the power fails, and failures over ½hour is very unlikely (and if it does happen, power wont be back till several hours).

 

[lbenson]

MORA99--thanks for starting a very interesting and informative thread.

I have a standard UPS which has kept alive a cable modem, router, and picaxe+simpleLan house monitor in a Nova Scotia. Several times last winter it notified me that the power had gone out (clearly the cable connection had not). But as in your case, 12V->120V->12V->5V is fairly inefficient, and it doesn't last too long (more than an hour, tho, as hurricane Bill showed me two days ago while I was at the site).

I would hesitate to say anything OT, but since because of your location you are probably not a native English speaker, and because you express yourself quite clearly and must surely have taken pains to learn English, you might want to know that the term you have used several times, "ofcause", is actually a two-word phrase--"of course".

Apologies if this is out of line, and thanks again for your picaxe contributions over the years.

 

[MORA99]

Ok, I took the UPS offline today afterall.
It contains 2 batteries wired in series I believe (Red to Black, ups connected to the free black and red).

From the label.
YUASA NP7.2-12 12V,7.2Ah
valve regulated
sealed lead-acid type
rechargeable battery
NON-SPILLABLE

The voltage is right now (30minutes after being disconnected) 12.80V.
12.97 on one.
12.77 on the other.

Bit wierd, if its series the voltage should be ~24 right ?
I have a bulk charger in the garage, and can try to hook some lamps or something to test if the batteries works.

lbenson : Noted thanks
English is not my native language, but due to our small country we really have to learn either English or German, often I find it easyier to find the right terms in English than Danish, probaly because so much technical information is in English already.
Of cause it does make it a bit hard to google for local resources.

 

[MORA99]

Odd, when I connected the battery back the voltage is 25.17
When the ups is turned on it goes to 27.20.
When powering off with a small fan connected the voltage drops to 19V for a moment, ups turns off and we are back at 25.21

Connecting a motor from a battery powered drill sends the voltage of 1 battery to 8.50V

I will go battery hunting

 

[moxhamj]

Connecting a motor from a battery powered drill sends the voltage of 1 battery to 8.50V

I think that means you have a dead battery. Maybe it will be simple to fix after all?

 

[MORA99]

Connecting a motor from a battery powered drill sends the voltage of 1 battery to 8.50V

I think that means you have a dead battery. Maybe it will be simple to fix after all?

Probaly yes, a new Panasonic battery is around 280DKK, a new noname 200DKK, so its fixable.
After opening the case it is also pretty easy to add temperature monitoring of the battery bank, all the high voltage stuff is nicely placed behind a metal plate behind the battery compartment.

Now I just need to find something else to do

 

[wapo54001]

I have had great luck replacing UPS batteries with (external) automobile batteries. This is especially useful when you want to extend the run time but don't need a higher capacity output.

Also, this way, you don't have to buy application-specific sized batteries.

I use an old APC to power my telephone and alarm systems which do not draw much power, but need to run for as long as possible. I extended the battery wires outside the box and connected a relatively inexpensive automobile battery. I tested the system and the battery was still running strong long after 24 hours had passed.

Some of the larger APCs require 24V -- 2 batteries in series -- still cheaper to buy car/motorcycle batteries.

You must give some consideration to hydrogen off-gassing, of course.

 

[MORA99]

I have had great luck replacing UPS batteries with (external) automobile batteries. This is especially useful when you want to extend the run time but don't need a higher capacity output.

Also, this way, you don't have to buy application-specific sized batteries.

I use an old APC to power my telephone and alarm systems which do not draw much power, but need to run for as long as possible. I extended the battery wires outside the box and connected a relatively inexpensive automobile battery. I tested the system and the battery was still running strong long after 24 hours had passed.

Some of the larger APCs require 24V -- 2 batteries in series -- still cheaper to buy car/motorcycle batteries.

You must give some consideration to hydrogen off-gassing, of course.

some of the "experts" on the various sites advise against this, but maybe its just myths ?
The problems range from not being able to charge a larger battery, timed shutdown (ie. turns off after 15minutes of no power), and running the inverter for longer than it was intended = overheat.

I was thinking of using some larger batteries, although AGM batteries is not cheap at high amp ratings (And the gassing stories does not sound good for closed space).
My UPS uses 2 batteries in series.

 

[wapo54001]

Automobile batteries charge to a higher voltage, so if you use the UPS charge circuit, the battery will never charge to the maximum. You could use an external charger if you feel the need . . .

If your UPS charger circuit is current limited, it may cause recharge to be slower, but eventually, it will get where it needs to be, won't it?

You can now buy completely maintenance free batteries which, I believe, are sealed so that no gasses escape.

Discharge curves are different for different batteries. Depending upon your application, you might want to research that, as well.

 

[Taniwha]

Automobile batteries charge to a higher voltage, so if you use the UPS charge circuit, the battery will never charge to the maximum. You could use an external charger if you feel the need . . .

If your UPS charger circuit is current limited, it may cause recharge to be slower, but eventually, it will get where it needs to be, won't it?

You can now buy completely maintenance free batteries which, I believe, are sealed so that no gasses escape.

Discharge curves are different for different batteries. Depending upon your application, you might want to research that, as well.

As an "expert" ( I sell UPS eguipment for a living) I would STRONGLY caution against using automobile batteries in a UPS, I don't know how good the "maintenance free" wet cell batteries are at containing and recombining the gassing. I do know that Automobile batteries have a completely different duty cycle to a UPS. Car batteries are designed for short duration high current loading, I.E. motor starting. If you are going to go the wet cell automotive route look for "house" batteries, these are at least designed for long slow discharge.

There are all sorts of issues with both recharging and floating these batteries, keep a close eye on battery temeperature at all times.
Sealed Lead Acid AGM type batteries (Yuasa, Panasonic etc) are designed for float applications and really are the best choice if you can get them. A good source of small (7ah) AGM's is your local security companies, they use them in almost every alarm panel.

 

[BeanieBots]

Concur with Taniwha.
Car batteries will not like being float charged for indefinite periods.
They will gas and then oxidise causing permanent loss of capacity quite quickly. (not to mention safety hazard). However, they are cheap, so replacing every few months could be comparable to replacing with 'proper' float batteries every few years.

Try to get friendly with an alarm installer.
They routinely replace perfectly OK batteries.
I got a whole bunch of 38Ahr AGMs from a server room UPS which had its 3 year battery replacement done recently.

I use them for a totally inappropriate PV system but I don't complain when they are free. So far they have given 8 months reliable service even with a daily discharge of ~50%. (don't try that if you pay for them!).

 

[Taniwha]

Concur with Taniwha.
Car batteries will not like being float charged for indefinite periods.
They will gas and then oxidise causing permanent loss of capacity quite quickly. (not to mention safety hazard). However, they are cheap, so replacing every few months could be comparable to replacing with 'proper' float batteries every few years.

Try to get friendly with an alarm installer.
They routinely replace perfectly OK batteries.
I got a whole bunch of 38Ahr AGMs from a server room UPS which had its 3 year battery replacement done recently.

I use them for a totally inappropriate PV system but I don't complain when they are free. So far they have given 8 months reliable service even with a daily discharge of ~50%. (don't try that if you pay for them!).

Beanie it might be worth your while talking to the local Telco or power authority engineers if you want PV batteries (or at least batteries for a PV system), down here in good ole NZ they regularly cycle out their 12v GEL batteries, typically after about 4 - 5 years on a 10 year design life battery.

 

[BeanieBots]

Indeed Taniwha, I'm also trying to get friendly with the local disability/mobility scooter firm as a good source of deep cycle batteries.

 

[manuka]

A nod & a wink used to be the case with all manner of ex. NZ Telecom discards, but such largesse largely evaporated ~1990 when they privatised. However -especially if carrying a dozen ale- you may be lucky!

 

[Taniwha]

A nod & a wink used to be the case with all manner of ex. NZ Telecom discards, but such largesse largely evaporated ~1990 when they privatised. However -especially if carrying a dozen ale- you may be lucky!

For Telecom these days you need to know who the local Patch holder is, a bit of a sore point at the moment of course.