Charging batteries in series

[EpicentrE]

I have used 2 and 3 cell packs up to 300mah in size, for over 10 years now. I've never used a balance charger, and I've never had a single problem with charging any of them. I think almost every (if not every) antweight roboteer will tell you the same thing.

[Sportball]

I understand I asked the question and I do not want to disagree but...

http://www.rchelicopterfun.com/parallel ... rging.html

I guess with 2 single cells I can either connect them in series and charge them as a 2 cell battery pack or according to this I can also do them in parallel.

[EpicentrE]

Mine's not to reason whether it's possible or not; electronics isn't my strong point and I'll happily let others field the question on that. I would simply ask why? When you have perfectly simple solutions, why are you interested in something that takes a very long guide with multiple images to explain? Which, even if it does work, is highly unlikely to give you any tangible benefits, given the difference between charging single ant-sized cells and massive multi-cell packs with balance chargers?

It's great that you're doing your own research and attempting to learn these things off your own back; that's commendable. But my advice to any new roboteer is to keep things simple. If you desire complexity, it will come as your knowledge and skills increase, but starting out with it is generally a bad idea.

[BeligerAnt]

Well, electronics is my strong point - I've been a professional electronics engineer for 25 years.

It is possible to charge cells in parallel, but there is one big caveat... The cells have to be pretty well matched before you start charging! It does say so in the article, but there's lots of verbiage to wade through to pick out the important bits.

All the Ohm's law stuff is somewhat short of the full story. Ohm's law applies to simple resistive circuits; batteries are not resistors, so to really understand what's going on you need to understand Kirchhof's Law and what actually goes on in a battery and how it behaves during charging and discharging!

As Scott (EpicentrE) points out, why bother? There is absolutely nothing to be gained in charging 2 cells in parallel versus in series. However, it is non-standard, more complex and more error prone. You need to remember to charge your 2 cells as 1 cell and at twice the required charging rate - plenty of room for error. You also need to make up some sort of adapter to convert your 2 cells wired in series in the robot to be wired in parallel for charging. Far simpler to wire them in series and just charge them normally, you don't even need to bother with a balance connection.

If you're really paranoid, connect your cells in series and add a balance connector. Then balance charge them in series, in a fireproof bag.

If you really want to charge multiple multi-cell packs at the same time, buy a multi-output charger that is designed to do the job properly.

[Sportball]

...... so series it is then.

Do you normally remove the tiny JST pH connectors and just use crocodile clips to charge or make up adaptors?

I made a charge lead today using JST pH sockets but they are bloody fiddly and probably won't last long. Good soldering practise though.

[joey_picus]

For the one JST-pH pack I have I made up an adaptor lead which works fairly well - I would avoid using crocodile clips because the dangers of short-circuiting LiPos in particular are great enough that you really don't want any risk of that happening (although then I'm not reknowned for steady hands at the best of times...)

Most of my batteries are plain 2 cell packs with no balance leads, and I've never had any issue charging those using a simple auto-detecting charger - if you're concerned about charging times, the best solution would be to mount the battery pack/cells so they can be switched out easily and have more than one set, so you can be charging one while fighting with another.

[Sportball]

Thanks for the advice. I have successfully soldered up some tiny JST pH sockets that allow me to plug in each 1c battery indepentally although they are in series on the robot. Both charged within 1hr so no problem there.

[Sportball]

I have left my robot on and drained the batteries after only 3 charges. From what I understand the charger looks for a voltage range before it will work. Can they be revived?

My idea is to order 2 more and charge 1 dead with 1 good one to get the voltage up enough so they can then be charged indepentantly, will this work?

[razerdave]

If its a lipo, then no. Don't try and revive them. Learn your lesson and get some new ones.

[BeligerAnt]

Depending on the charger, it may be able to charge them. And if they have over-discharged due to being left "on" but not running the cells should be recoverable. Charging a "good" cell in series with a "bad" one will kill the already wounded cell!

There is a minimum voltage of around 2.5V per cell. Below this voltage they don't take kindly to being charged at a (relatively) high current. Normal charging current is around 1C where C is the capacity, so 100mA for a 100mAh cell, 200mA for a 200mAh cell and so on.

If you're lucky, your charger will detect the voltage as being too low and will apply a very small current to the cell to slowly charge it, then when the voltage reaches 2.5V automatically start charging at the proper rate.
If you're not so lucky, the charger will detect the voltage and just sulk and refuse to charge the battery at all.
If you're really unlucky, the charger will not detect the voltage and will try to charge the battery and kill it - if this happens, get a different charger!

Since it sounds like your charger is in the "just sulks" category, you could try applying a very small current ( less than 0.1C) to the cell and monitor the voltage, stopping when the voltage reaches 2.75V or so. Then you should be able to charge the cell fully using your charger.