BATTERY CAPACITOR?`

#1
Hi All

Not directly related to a Picaxe but I am hoping someone can help.

I have a battery (29.2v DC) and its made up of Lithium cells. Inside the battery there is a capacitor across the positive and negative wires which then lead directly out of the battery. The capacitor is a 35V 220uf.

Please can some one tell me what is the purpose of the capacitor in the battery?
Also, would the presence of the capacitor positively effect the outcome of an EMC test on the device the battery powers. ( A brush less motor and controller)

Thanks

JPU

Correction made: cap size is 220uf.
 
#3
@JPU - what size - amp capacity - are the series lithium cells? In any series string on discharge some cells can become reverse charged and lithium cells also don't do well when discharged below 3v or so... so usually a BMS is used [Battery Management System]. With coin cells this becomes less of an issue as the energy is small when things go wrong... ;-0
 
#4
Hi All

They are LIFEPO4 cells 8S2P 18650. The pack is nominal voltage 25.6V3AH

The machine is basically a brush less motor and controller.

The pack originally had the Capacitor added to the output wires within the case, However over time and testing the battery has become removed fromthe case and the capacitor remained (got forgotten about!!) in the case. All testing has been done on the battery and motor over months (this was field testing as opposed to lab testing) just to see where and what would break or be a problem. The tool has worked well and I am just going to get EMC testing on it. Its all arranged and its off to TOBY labs in China tomorrow!

However, last minute jitters! checking it all over and I noticed the cap!! Last year I had something fail on EMC testing and it was a nightmare to find out what was causing it to fail. So I'm EMC scared!!



Thanks

JPU

PS. Any other pre-packing/posting tips will be greatly appreciated which help to get through the EMC test!
 
#5
The capacitor seems likely to be for a small EMI reduction... some 18650 cells have internal protection - often you can tell they are a little longer and have something added on the + end - this could replace the usual BMS circuit board seen in computer battery packs and such... i don't know what voltage these internal protectors are rated at... good luck with it...
 
Top