There are two phenomenon that batteries experience during charging - 1) slight differences in the resistance of the batteries and 2) slight differences in the amp hour capacity of the batteries.
When batteries are installed in series, each battery receives the same amount of current. Inconsistencies in the manufacturing processes, or aging can change the internal resistance of the different batteries.
A simple Ohms law calculation shows the increase in voltage that occurs as resistance changes. During the charge phase the weaker batteries show increased voltage due to the higher resistance. This is additive over time so every cycle, if left uncorrected, drives the batteries further out of balance.
The amp hour capacity decreases on the weakest battery so it becomes fully charged more quickly and before the other batteries. Once it reaches full charge, there are no more electrons available for transfer so all that is left is to electrolyse the water. This emits hydrogen gas. The hydrogen gas escapes and the battery starts losing its electrolyte.
Once the electrolyte vanishes, the battery fails.
The effect of these minor fluctuations becomes apparent over cycle life and time and drive the batteries out of balance with every cycle. It takes very small corrective action by putting energy into low cells (additive) or taking energy out of high cells (subtractive) to keep the batteries in balance.
During the charge cycle the weaker battery increases in voltage but during the discharge cycle the weaker battery decreases in voltage with respect to the other batteries in the string. We describe this as the weaker battery becoming spongy and not stiff like a good battery.
Effective Battery Balancing extends the life of your battery pack. Battery monitoring pays for itself over time plus gives the immediate satisfaction of knowing precisely the condition and capability of the energy system on board the boat.
Meters that measure voltage of the battery pack are not representative of the true energy remaining in the weakest battery, which determines the strength of the entire system.
The Electric Propeller Company
625C East Haley Street,
Santa Barbara, CA
93103
805 455 8444
james@electroprop.com
www.electroprop.com
For years I have used flooded cells in series/parallel circuit and never noticed this. Having completed the motor installation in my boat I have just received my set of four 12V AGM 100 AH batteries and charger, Lifeline batteries and Qui-Q 48V charger with temperature compensation.Off the truck the individual batteries terminal voltages varied by about 60 mV, not unexpected. I put each through a short discharge cycle and then wired up in series to charge them. At the start of the charging cycle I noticed quite a variation in terminal voltage of each battery which I expected. What I did not expect was that as the current tapered off, less than three amps, the differences in voltage between individual batteries increased quite noticeably. I would have expected that each battery would have accepted the charge it needed and the voltages would have become close to the same for each battery.Would it have been better to charge each battery individually before putting them into series?After putting the battery stack through a few cycles should the difference in battery voltages go away?Any thoughts?thanks,John Acord
Posted by: James Lambden <james@electroprop.com>
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