Charging Lithium-ion Batteries

I have just purchased some genuine Sharp Viewcam BT-L12 Battery Packs, at a very reasonable price.

3.6 Volts 3900 mAh ... and made in Japan!

I plan to use four of them in series, to produce a nominal 14.4 Volts, which will then be regulated down to 12 Volts.

So far, so good ... but I need some advice regards charging.

Each pack has three terminals | + | |T| | - |

I know that |T| connects to a thermistor, to protect against overheating when charging ... But ... what do I do with FOUR of these |T| connections ?

Grateful for any guidance from those that know how.

MichaelG.

Hi Michael

I found the following Google serch terms found a number of papers....

charging lithium ion batteries thermistor

copy the whole line above.

Cheers

John

Edited By John McNamara on 08/08/2012 14:53:14

John,

Thanks for responding:

I was looking at some of those sites last night ... the Panasonic document is particularly good.

The problem appears to be that equipment manufacturers make a special charger for almost every product [and they don't disclose the details].

A Thermistor is a temperature dependent resistor so [presumably] with four connected battery packs to charge; I could connect the |T| terminals in series, or parallel, or a mixture; or just rely upon one thermistor to monitor the four battery packs. And, of course, I don't know if eveyone uses the same values !

There is probably a design for a "universal" intelligent charger out there somewhere ... but I have yet to find it.

Thanks again

MichaelG.

Hi Michael,

I have made an 'all chemistry' univeral charger, but it still needs some tweaks to its programming.

Lithium-ion batteries are a challenge, not so much because they have very sensitive and complex charge requirements, but because if you get them wrong they can catch fire in a spectacular way (as per some laptops a few years ago).

You need to monitor all four thermistors separately and are probably safest charging all four pack separately. You are not just using the thermistor to detect end of charge like with Nicads - but to watch out for one of the cells going into serious overheat.

Neil

Thanks Neil ... that's pretty much what I feared.

Dire warnings duly heeded !!

Trouble is, I was planning to make up two "four packs", and that will involve a lot of separate charging.

MichaelG.

Just now two 12V 6.8 Ah Li-ion packages are under way from China. There are lots of single cells available, but I opted for the package because I thought that charging is not so easy. The packages come with their own charger. Oh - almost forgot - all from Ebay naturally:

http://tinyurl.com/c9jjmez

I hope this will not result in a spectacular firework!

Greetings, Hansrudolf

Noting Neil's advice ... it seems that the only safe approach is to charge the individual units separately.

Given the typical price of the equipment manufacturers' proprietary chargers; a bank of these would appear prohibitively expensive.

But there is hope: I have just found a very informative datasheet for the SC824 device.

A four-bay charger, using four of these on a common supply, looks the way to go.

MichaelG.

Edited By Michael Gilligan on 09/08/2012 08:27:48