Best Charge Algorithm for my expanded system.

CALLD · February 2015

I've just added 610watts PV to my system and gone with a 70A Victron MPPT charge controller. 1st day running it has seen peak charging power reach 1614w @ 25.9V & 62.3A (2000w load on inverter for testing). I'm pleased with the results, but am trying to decide on the best charging algorithm now based on the fact that I now have an over-sized array for my batteries.

Battery manufacturer recommends an absorb voltage of 29.2 to 29.6V @ 25 Degrees C.
Charging Current should be between 26 and 52A during bulk, with absolute maximum bulk current of 78A

The Victron charge controller is defaulted to charge until 28.8v with a varying absorb time of 1 to 8 hours depending on the battery voltage at dawn. The lower the start voltage at dawn is the longer it will stay at absorb. The dealer who sold me the Charge Controller AND AGM batteries has advised me to keep absorb @ 28.8v. Can anyone tell me which is better - dealer advice or what is stamped on the battery? I have seen in my handbook of batteries that VRLA/AGM batteries require varying absorb voltages depending on depth of discharge - with greater DOD's required higher absorb voltages for optimum cycle life. Currently I cycle them using 30%DOD daily (100-70-100% SOC Daily).

BB. · February 2015

In general, solar power systems usually require charging voltages on the higher voltage side of the suggested ranges because you simply have fewer hours of charging current available (limited by the bright thing in the sky) than somebody with utility power (or willing to run a genset for many hours per day).

28.8 Volts is the traditional/typical charging voltage for AGM batteries (at ~25C). But if your battery mfg. recommends a higher voltage--I would be much more likely to suggest the higher voltage.

Because AGM batteries are sealed an you cannot measure electrolyte state of charge (actually, from what I have read, it may be possible to open the cell cap on some AGMs and pull a little a tiny drop of electrolyte and use an optical hydrometer (refractometer) to measure the SG)--You have to look at secondary indications for state of charge.

Resting voltage (3+ hours of no charge/discharge), case bulging (blowing up like a balloon (from gas pressure during charging). Battery venting (excess gases released by over pressure valve--Not a good thing to do). Also, there is a small catalyst in the top of the cell--It gets hot as the Hydrogen+Oxygen recombined and generates heat--Too high of gassing (high charging voltage) can cause the catalyst to overheat.

Monitoring the charging current and looking for the "tail" (as battery charges during absorb stage--constant voltage charging--You will see the current fall and eventually "plateau" (current will stop falling). That is usually when the battery is near 100% full. However, if the charging voltage is too high--That level of current could still damage the battery.

I really do not have a good answer for you (as I have said before, I am not a battery engineer). I would talk to both your battery supplier and the mfg. and tell them your usage (solar RE with limited daily charging times) and see if they can give you more guidance.

The problem is that even Battery Mfg. have had to change/update their charging specifications as they gain experience in particular applications. And sometimes, there is disagreement inside the company and specifications are released that are actually less than optimum (i.e., "wrong") for the application and hurt battery life.

I am sorry I do not have a good answer for you... Perhaps some others here can give you better/more useful information here.

Also, can you let us know the Mfg. Name and Model of your AGMs? Perhaps somebody here has experience with them.

-Bill

HandyBob · February 2015

The above was a good answer, but a bit technical. I know some AGM outfits are recommending higher voltages, but do they really have experience with what we are doing with their batteries? Not very likely. I have seen problems with off brand AGM's charged at higher rates. Whether this was voltage caused or just crappy manufacturing I cannot say. Personally, I won't go higher than 14.4V or 28.8V no matter what they say. Batteries are batteries.... If you charge at 28.8V until the input amps drop to below 2% of the C20 rating, they are full. After that, the float rate is not critical and hours of trickle charging will never hurt them. Don't drive yourself crazy.

CALLD · February 2015

@ BB, Yes there is so much to think about - I've also tended to lean towards the higher voltage especially after my previous experience with flooded batteries, but with AGM/VRLA batteries more damage can be done due to over charging than under-charging so perhaps this time I may lean towards the lower voltage recommended by the dealer and by HandyBob on this thread.

I wouldn't try taking SG readings on these since they appear to be very well sealed and are still under warranty. The brand is SonX, and they are mostly found in South Africa.

With my previous charge controller I used to have it set to 29.4V so it was in the middle of the battery manufacture's recommendations, but there was always something that concerned me once the current tapered off to <4Amps - the battery voltages would go a bit off balance (one battery reading 14.5V and the other reading 14.9V). It wasn't always the same one showing the higher voltage, they sort of took turns. I'm not sure why this happens but it always happens when they get close to 100%. Anyway with the absorb voltage set at 28.8V the voltages are closer together by the time the current tapers off to <4Amps (14.3 vs 14.5V is what I've observed). I was obviously concerned the first time I observed the difference, however under load the difference is less than 0.02V per 12V battery, during bulk charge the difference is also about 0.02V per battery.

niel · March 2015

calld,
i'm a bit late in reading this and i have agms here. i'd guess the batteries were starting to overcharge when voltages went goofy. it may or may not have gassed some, which if it did you've lost a bit of capacity. i must mention that charging voltages are for 25 degrees c and if the batteries are warmer than that you need a smaller voltage for charging. when colder it goes upward some. some ccs are able to compensate for temp variations and it's recommended that if your cc can accommodate such variation that you should use it. you need to know the battery manufacturer's temp compensation rating if choices are allowed in the programming of the cc.

if the cc has no compensation you could just lower the charge voltage .2v-.3v.on the manufacturers recommendations if ambient temps are consistently higher than 30 degrees c for daytime highs on your 24v bank. many batteries go (.005v per cell per degree f above 77) lower in voltage when the temps are above 77 degrees f (25 c). it varies per manufacturer and battery type so you should try to look it up, but a good guess at 5mv as i indicated could help until you know for sure.

as to manufacturers recommended charge voltages, the midpoint is a good place i'd have put it, but in suspecting possible overcharging i'd move the voltage down to the low end of their recommendations. do observe the float voltages too as you may wish to adjust that down a tad too. i would be curious of voltage readings you might get after 3+ hours of no loads or charges (at rest) and see the voltages on each battery then. check periodically like every few days over a few weeks to see if the same whacky voltages pop up on the batteries and to see if the at rest voltages are consistent and good. if it is still doing it with those voltages popping high randomly, but improving, then try lowering it another .1v to see if that cures it.

Best Charge Algorithm for my expanded system.

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