24V AGM batteries in series at 13.40V and 13.63V. How to balance?

[soylentgreen]

I have two 145AH AGM 12V telecom batteries I use for emergency backup only - these batteries sit at float for 364 days per year, as they are connected to a UPS.

On charger at float stage, one is at 13.40V and the other is 13.63V, so an imbalance of 0.23V.

How out of balance is this?

What's the easiest way to do a one-time balance manually? I don't think there's any justification for adding more hardware to this system, since they are not cycled regularly.

 

[Rednecktek]

The easiest way? Disconnect the battery bank from the system, then throw them in parallel for an hour or so. Any imbalance after that should be not worth worrying about. Even a quarter volt on AGM is negligible so unless you're really bored....

 

[pollenface]

Invest in a HA01 balancer

 

[sunshine_eggo]

The easiest way? Disconnect the battery bank from the system, then throw them in parallel AND CHARGE for an hour or so. Any imbalance after that should be not worth worrying about. Even a quarter volt on AGM is negligible so unless you're really bored....

Revised.

Putting two 12V in parallel will transfer very little charge especially when done for only an hour or two. Any induced current raises the low voltage and lowers the high voltage further reducing the delta and the current. Also, as lead acid voltage has a tendency to settle following a charge, the voltage differential is ever-decreasing thus providing even less potential for current to flow between them.

Putting them in parallel and charging will create a reservoir from which the low battery can draw much faster than simply just being in parallel.

The only chemistry that benefits strongly from non-charging parallel wiring is 3.7V Lithium. The strong voltage to SoC correlation results in negligible "settling" and encourages charge transfer until they are very nearly equal.

For an operational system, options:

• Charge the lower voltage battery individually while wired in 24V.
• Drain the higher voltage battery individually while wired in 24V.
• Invest in the aforementioned HA01 or similar balancer for ongoing maintenance.

NOTE that the most important voltage deviation is at peak 24V system voltage. If you're charging to 28.8V, imbalanced batteries could see the higher voltage battery ABOVE the specified 12V limit (14.4-14.8V typical). This can damage the high battery and undercharge the low.

If you observe your 12V are above spec, lower your system absorption voltage such that the highest voltage one is not over spec.