# Uneven charge across string

Discussion in 'Home Power and Microgeneration' started by David Williams, Jul 11, 2007.

1. ### David WilliamsGuest

-> We have 12 2volt AGMs which are manufactured in 6volt blocks each
-> containing 3x2v cells. Each cell is (supposedly) completely isolated
-> and has its own external terminal posts.

-> These are all wired in series and connected to a Studer charger.

-> charger
-> \\
-> Pos - 6v-6v
-> Neg - 6v-6v

-> The two blocks at the front, connected to pos and neg respectively,
-> seem to be absorbing more of the voltage.,

-> Voltage across each cell in the forward blocks during charge is around
-> 2.4-2.5v, whereas at the two blocks to the rear it's around 2.2-2.3
-> volts across each cell.

This often happens when cells are connected in series. Basically, all
the cells get the same charge while they're charging, and lose the same
charge while they're discharging. They *should* all stay balanced. But,
in reality, there's always some leakage of charge, and this varies from
cell to cell. After many charge-discharge cycles, this leads to some
cells being fully charged when others are almost discharged, so the
voltages are unequal. It also reduces the effective capacity of the
battery, since its voltage drops substantially when some cells are
discharged, even though others still have plenty of charge.

If you can disconnect the cells from the string and fully charge each
of them individually, using an appropriate low-voltage charger, then
put them back into the string, then, if the cells are in reasonably
good shape, the battery should work properly again for a while.

dow

2. ### George GhioGuest

Clarity is required for concise answer.

System voltage would be nice. If they are all wired in series then you have 24V

Sounds like series/parallel
Are all the wire lengths the same.

i.e.

Pos - *6v-6v
Neg - 6v-6v*

Where the actual connection is marked as * as apposed to
Parallel strings should be avoided unless there is no other solution.

Are all the batteries the same capacity and age?

What type of connectors are used?

3. ### George GhioGuest

Ok. It used to be that battery connections were soldered, not bolted. If you find that the problem
persists it sometimes helps to but the reluctant batteries at the end of the string rather than the
middle.

5. ### RW SalnickGuest

David Williams brought forth on stone tablets:

Maybe you can explain why, when I check an overcharged car battery, it
is almost universally the cell closest to the positive terminal which is
lowest on water, and the cell next to the negative terminal which has
the most water...

bob
s/v Eolian
Seattle