Charging li-ions in series
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I know there are a ton of topics on charging lithium Ion batteries and I have been doing some searches to find an answer to my specific question but can't seem to find the answer. Maybe this is just common sense and everyone knows it. Anyways I have 2 7.4v li-ion battery packs (both have protection circuits on them) and I want to hook them up in series to obviously obtain 14.8v. So I'm going to use a 14.8v rated battery charger, but what if one of the 7.4v battery packs is charged more than the other before I charge them together for the first time? Cause if it is charging and one of the battery packs is at a slightly higher voltage than the other then the protection circuit will shut off charging while the other battery pack is only say 3/4 the way done. Will this cause any problems or will all be ok? Thanks in advance.
Re: Charging li-ions in series
I would advise against running two sets of Li-Ion batteries in series because you would be using them in a condition that the PCMs are not designed for. Although each PCM would ensure that their respective cells stay level and don't get over-charged or over-discharged, there would be no system in place to keep the sets balanced with each other.
In the charging example you described, the first pack would be removed from the circuit, so no charge would reach the second pack. But there are also some other things to consider that could be potentially dangerous, such as placing a 14.8V charger across two 7.4 packs. You wouldn't be able to control how that 14.8V (or rather 16.8V, which is the voltage output for a 4-cell charger) is distributed across each pack, so it could cause them to charge at different rates, or do something much worse.
There is also the concern of how this would affect discharging. It's hard to say what would actually happen if the voltages for the two sets start to deviate during operation, but with Li-Ion batteries I personally would not want to take that risk.
The simplest and safest solution would be to purchase a PCM that is designed for 4 cells in series, and wire your batteries onto that. Here is one from batteryspace.com that should suit your application (5A max drain limit should be sufficient).
In the charging example you described, the first pack would be removed from the circuit, so no charge would reach the second pack. But there are also some other things to consider that could be potentially dangerous, such as placing a 14.8V charger across two 7.4 packs. You wouldn't be able to control how that 14.8V (or rather 16.8V, which is the voltage output for a 4-cell charger) is distributed across each pack, so it could cause them to charge at different rates, or do something much worse.
There is also the concern of how this would affect discharging. It's hard to say what would actually happen if the voltages for the two sets start to deviate during operation, but with Li-Ion batteries I personally would not want to take that risk.
The simplest and safest solution would be to purchase a PCM that is designed for 4 cells in series, and wire your batteries onto that. Here is one from batteryspace.com that should suit your application (5A max drain limit should be sufficient).
Re: Charging li-ions in series
Yeah I figured that it wasn't a good idea. I just wanted to make sure. Thanks for the reply!