Damage From Charging SLA Batteries In Series 1

[shshibby]

Greetings all,
I came across the engineering forums while doing some researching on battery charging. I have had limited success finding solutions to my specific problem and I am hoping that some members here have knowledge/experience in this area.

My company manufactures a product which uses three 12V, 18 Amp Hour AGM sealed lead acid batteries. The batteries are connected in series to provide 36V for the system. The batteries are the main source of power for the system which is often used where access to AC power is limited. However, in situations where AC power is available the system can be used while charging. Continuous current draw on the batteries during normal operation ranges from 1A to 7A. Momentary current draw can spike as high as 60A to 80A but will settle below 10A within 30 seconds.

Our charging solution to this point has been commercially available 36V battery charges. We have used two different chargers - originally a 3A charger and more recently a 4A charger. The chargers are from different manufactures, both are multi-stage “smart” chargers designed to be left connected indefinitely without harming the batteries.

While the charging solution works the majority the time, a growing number of customers have reported condensation or liquid inside the battery compartment, “steam” coming from the system, extreme heat on the outside of the unit and in some cases batteries becoming bulged. In most cases this has occurred while systems were off (no current being drawn from batteries) and left to charge for 6+ hours. We believe this problem is the result of overcharging caused by a voltage imbalance between the individual batteries but we are not sure. Having used chargers from two separate manufacturers we do not believe defective chargers are to blame.

To prevent this problem we select sets of thee batteries from the same manufacturer/lot. We also charge the batteries individually using 12V chargers immediately prior to installation to ensure the batteries are balanced. However, despite these precautions the problem continues to occur and always results in catastrophic/non-recoverable damage to the affected system.

Please note the three batteries are installed in the system such that they cannot be accessed or removed by the user and they cannot be disconnected to allow for individual charging.

I have considered two possible solutions:
1) Devise a method to charge each 12V battery individually while all three batteries remain connected in series
2) Devise a means to actively balance the batteries to prevent any given battery from becoming overcharged

I welcome any and all insight, guidance or recommendation you may have, thank you.

 

[itsmoked]

Nasty!

Point one: While charging each battery fully @ 12V while building the pack is good there is something more important. It's the battery's capacity that is much more important when building series packs. It would be considerably more useful for you to get or build a capacity tester. The general aspects are to fully charge a battery and then load it to an appropriate level for a given amount of time (energy) and then measure the battery's State of Charge (SOC). From this you get the battery's capacity. You want all the batteries in the series pack to be within a few percent of capacity.

If they aren't then repeated pack cycles will 'wear out' the cells of one of the members and those cells will over-discharge and over-charge repeatedly resulting in eventual heating and cell destruction - just what you're seeing.

Second point: SLAs are VERY sensitive to temperature in all usage aspects. Really good chargers use temperature sensors placed on the batteries. All charging operations should be controlled by what the measured temperatures show. It is vital to the dynamic selection of voltage-points the charger controls to.

With the last in mind, I suspect your pack is suffering by having the various batteries physically contacting each other in a completely non-symmetrical manner which is resulting in cells or indeed a whole battery running hotter than the neighbors. This changes the charging voltages and hence the charging stage switching points for all the various cells, something that regular chargers have no way to handle so mis-charging and cell wear-out and eventual (possibly rapid) complete cell failure is happening.

It's not what you probably want to hear but your pack's physical arrangement is probably not helping with balance. The solution is to try to do something to make them all play together better. You want them all the SAME temperature. This isn't as much of a issue if everything happens slowly. Slow charging would allow a more uniform temperature across the pack so some apps might not care about having three batteries in close proximity, yours, not so good. If you can do something to help the thermal picture it would help greatly. Probably it's not easy to increase the battery space in your product by a few inches to give the middle battery a chance at a normal life? Maybe a half inch and fans to move the air between the batteries? Perhaps insulating 'just the right amount' the sides of the outer batteries so all batteries keep the same temp during all operations? Might be crazy but perhaps smaller batteries so there can be space will actually result in better performance.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[mikekilroy]

very perceptive idea Keith! hopefully gave op a whole 'nother angle to consider. worth ps!

building on same temperature thought.... actual charge/float/equalize VOLTAGE is also very dependent on AMBIENT temp....

example is my ezgo 36v golf cart.... i just had to replace the micro controller pcb in my charger... found they did not take into account ambient temp, which means in my case i should charge to about 44v@80F, 45@65F, and 47v@45F.... i offered the replacement pcb mfgr the idea that they could change charge voltage based on ambient temp and thus have the best charger available.... will see if they do it.

http://www.KilroyWasHere<dot>com

 

[itsmoked]

Thanks Mike. While ambient is important it really is ALL about the BATTERY temperature not ambient. You can imagine ambient batts being heavily charged and having them heat up because of it, hence the sensor should be attached to the battery(ies). If they happen to be ambient or are charging at such a low rate that they remain essentially ambient all the better. If it's a cheaper charger or should I say less expensive (as it at least has a sensor) and has only an 'ambient' sensor that's way better than nothing and has a much better chance of working adequately, especially if the battery is a single unit with lots of air circulation like a car battery.

Hallo shshibby?
Are you getting this? Is it clear enough?
Are you there?

Keith Cress
kcress -

http://www.flaminsystems.com