Diode Question

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RustyH

Senior Member
Good Evening All,

Probably a really basic question, but I'm not sure how you go about calculating a spec for a diode. I currently use an Imax B6 charger to charge two 6v 12Ah battery wired in series (12v).
At the moment I disconnect and take the batteries out and charge them, but I want to do it in situ. I plan to do this via a DC plug type connector, but I want to prevent flow from the battery to the female connect when not charging. What type of diode would I need?

Also, is there a simple circuit or device that would automatically swith power to the circuitry off while charging, and back on when not charging?

Many thanks
Russ
 
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inglewoodpete

Senior Member
I would expect that the reverse current into the Imax B6 to be in the microamp region. Have you measured this current when the charger (or its mains supply) is turned off? Inserting a series diode would upset the charging algorythm and is likely to result in undercharging.
 
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inglewoodpete

Senior Member
I was thinking along the lines of a zero volt drop solution using a relay or a MOSFET. I haven't got the time right now to investigate whether a MOSFET's substrate diode would defeat its purpose. Worth investigating though.
 
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RustyH

Senior Member
Thank you for the replies.
Never knew there was such high losses in diodes, and see what your saying about upsetting the Imax.
Would the simplest method be to put a switch on the positive line of the charging connector that would isolated the connector when not being charged?

Infact, am I over thinking this and do I really need to isolate the charging connector? Can you get a shock from a 12v 12Ah battery or can the batteries get damaged if the connector being wet and shorted (if left outside).
 
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AllyCat

Senior Member
Hi,
RustyH said:
Infact, am I over thinking this and do I really need to isolate the charging connector? Can you get a shock from a 12v 12Ah battery or can the batteries get damaged if the connector being wet and shorted (if left outside).
Click to expand...
No, 12 volts is unlikely to give you a shock. Generally, anything less than 50 volts is considered "Low Voltage" (and not dangerous). But water and electricity seldom go together well, so you might have issues with corrosion (electrolysis), etc. in the connector. Also, the current from a 12 Ah battery is potentially dangerous; a metal bracelet or (wedding) ring accidentally connecting across the terminals of such a battery could do you real harm.

You haven't stated the type of battery, but "6 volts" sounds like 3 x Lead Acid cells in series? The charging requirements of Lead Acid batteries are well documented, so you probably could adapt the charging circuit to accommodate a standard silicon diode drop (not just ~700 mV but with a significant temperature coefficient). I've never been very impressed with the specifications of Schottky diodes, except in very particular applications. For other battery technologies and/or an off-the-shelf charger, I would suggest a relay, which can probably be energised by the charger itself. Obviously the relay contacts must be capable of carrying the charging current, but the primary "wear-out" mode of relay contacts is when breaking a large current flow (which might also occur during "contact bounce" when the relay is being activated).

Certainly MOSFETs are "bi-directional" (for example as used in Level-Shifters for the I2C Bus) but I don't know if they are truly symmetrical (even if excluding the substrate diode). Bipolar transistors are also bi-directional, but their reverse characteristics are highly inferior to the (intended) forward direction. The reverse current gain (hfe) is normally less than unity and the (reverse base-emitter) breakdown voltage little more than 5 volts (actually quite a decent zener diode reference sustitute). A saturated bipolar transistor (PNP) might make a simple and reasonable compromise replacement for the diode or relay.

Cheers, Alan.
 
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