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Good day everyone,

I recently purchased a “ZOP Power 7.4V 2200mAh 2S 35C Lipo Battery T Plug” for a project. I have designed a circuit to charge the battery, but now I am wondering if it is necessary to ensure the batteries are balanced when charging. I am using an 18V, 10W solar panel to charge my battery through a buck converter which controls the charging modes (CC/CV), the battery is connected to an RC car to extend running time. Through research, there are websites and forums with completely conflicting views, some say it is very important to balance charge batteries in a pack, whereas others state since there are only 2 cells present balanced charging can be ignored and the battery can be charged normally in series.

There doesnt seem to much information on these batteries, but this website contains some basic information on it: https://www.banggood.com/ZOP-Power-7_4V-2200mAh-2S-35C-Lipo-Battery-T-Plug-p-992114.html?cur_warehouse=CN#jsReviewsWrap

I look forward to hearing peoples advice and opinions!

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If the cell capacities are well matched and they have equal charge then they shouldn't need to be balanced while charging. So...

When you get the battery, measure the voltage on each cell via the balance connector. If they are within 0.03V of each other then it should be OK to charge the pack via the T connector. When charging for the first time, monitor the voltages to make sure that neither cell goes above 4.23V.

At full charge both cells should be within 0.01V of each other. If one cell is higher then discharge it with a 100Ω resistor until it is a bit lower than the other one, then let the battery rest for a few minutes and measure again, repeating until the resting voltages are equal.

A well balanced Lipo pack should remain balanced for its lifetime, unless it is damaged or abused. However a cell may (rarely) develop a partial short circuit which causes it to slowly discharge. Therefore you should measure the cell voltages regularly to check the balance. A 'leaky' cell will continually go out of balance, so if you find the pack needs regular balancing it's probably best to discard it.

The more cells there are in a pack the more chance there is of one cell getting badly out of balance without affecting the total voltage enough to notice, but even a 2 cell pack can be affected. I have a brand new 2S pack on my bench right now which has one cell at 3.75V and the other only 1.78V. If I tried to charge this pack the higher cell would go above 4.23V and blow up well before the total pack voltage reached 8.4V. Unfortunately at less than 2V the other cell cannot be revived, so the battery is useless.

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  • \$\begingroup\$ Thanks Bruce, I was hoping for a good answer, but this may be a perfect answer! I will start running tests to see if balanced charging is needed. Kind regards \$\endgroup\$ – Zee96 May 16 at 10:22
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Balancing them when it's not strictly necessary won't cause any issues.

NOT balancing them when it IS necessary WILL cause issues. Dangerous, fire or explosion based issues.

I'd take the safe approach if I were you.

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You should allways keep the cells balanced in order to mantain a balanced power draw from each cell.

If, by come reason, one of the cells starts having under voltage, you might overload the other risking a ball of smoke...

You know what they say: Balanced LiPo is a healthy LiPo!

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