Flooded Acid Charge Rate

bigbrovar · December 2019

Hi guys,
I changed cities and decided to restart my offgrid setup from scratch after selling of components from the former system (cheaper than transporting) I have 9 x 335w Yingli Solar panels, a Victron 150/100 mppt controller and a Axpert 3kva 24v inverter. I planned on getting a 4 24v 400AH battery but I am undecided on which of FLA and AGM to go with

I naturally prefer flooded acid battery due to their longivity when compared to AGM. Howvever my issue is the charge current.

Most FLA batteries are rated to be charged at 10% - 15% OF c20 at bulk stage or about 40A - 60A in my case. Based on my configuration, I might be getting close to 90A -100A during peak generation and my fear is this might affect or damage the flooded acid. I plan to offset the current dgoing into the battery by using oppotunity loads but that might not be enough to bring the charge current to the rated level required. On average my night load is about 90Ah - 100Ah so those batteries would be cycled at about 25% dod on a daily.

I quess my question is, can I, should I still go for the flooded battery? I might be charging the battery at 20% of c20 or even more. The alternative is to get an AGM battery which would cost a lot more and probably not last as long but which would be able to take everything my system throws at it in way of charging.
I want to make the most of my shallow depth of discharge and get the most from my battery while getting the most bang for my buck. Right now flooded allows me do this... but the charging rate issue.

mike95490 · December 2019

So, this is not the "right" answer, but I'll throw some real life at you.

The larger the difference between battery voltage, and charging voltage, along with the internal resistance, sets how much current the battery will "draw". By lowering the voltage, you can reduce the amount of current it pulls from the charge source.

if you hit a deeply discharged battery with a 15V 300A charger, you might get 200A for a minute or two. But as the battery voltage comes up, the charge amps start to taper off. Some controllers have a Max Amp limit you can program.

jonr · December 2019

As Mike suggests, a charge controller with current limiting is the logical solution.

Rolls recommends up to 20% (which is 80A) with heating listed as the concern if you go higher. My guess is that you could sometimes go to 100A if there is a way to limit temperature.

mcgivor · December 2019

With 9 × 335 W panels you would likely see a maximum of 80A or 20% charge rate, this would only occur at peak sun in optimal conditions with discharged batteries. Because the array will ramp up as the sun climbs higher, the amount of time you would actually see that amount of current would not be extensive because the voltage would have increased to the point where current begins to drop.

Flooded batteries can take 20% charge rate for a limited time, the recommendations are generally based on grid charging from completely discharged state over an extended period of time. In a solar application it is probably benificial to use a higher current due to the limited window of opportunity.

If there is a need or concern, a controller with limiting capability could be used as stated above, or alternatively a load could be applied to the inverter to divert some current. Personally I think having 20% charge rate available with a 25% discharged bank would be fine and extremely benificial during less than ideal conditions.

BB. · December 2019

Midnite Classic and "Kid" versions both support a remote shunt "Wizbang Jr" to allow control of charging current to the battery bank:

https://www.solar-electric.com/residential/charge-controllers.html?manufacturer=39&nav_pwm_mppt=411
https://www.solar-electric.com/midnite-solar-whiz-bang-jr-current-sense-module.html

Outback may also have an add-on that can measure/limit charging current--But I am less sure how well that works/how easy to configure.

If you size your battery bank for 2 days of storage, and 50% maximum discharge... That allows you to recover after a day or two with 10%+ rate of charge (typical 75% back to 90%+ charging).

If you have a very small FLA bank, (1 day of storage, 50% discharge), it usually takes closer to 2 days of sun to fully recharge the bank from 50% to 100% SoC. AGM and Li Ion can charge faster--So smaller AH battery banks can be OK as they will take more current to recharge back to full.

-Bill

Estragon · December 2019

I have pv capacity to theoretically put ~100a into a 370ah flooded bank. Being normally colder than 25°C, effective capacity runs more like 300ah, so about C/3.

I've never seen anything like 100a in actual use, but having the extra pv is useful on lightly overcast days.

bigbrovar · December 2019

Thanks guys for the inputs. I went for the flooded acid batteries 200ah x 4 at 24v which would give me a bank of 24v 400AH I intend to discharge them over night at 90AH Max (Based on energy audit) so by morning I would need to put back about 90AH +- 10AH back to the bank.

I am hoping that the system would be past the absorption stage before the panels hit peak generation mostly due to the size of my array (3kw). The opportunity load in the house is a fridge and freezer and both use less than about 10A combined. My fear is for the relatively high current not to damage the battery over time especially for period where the battery was drawn down to 50-60% (due to bad weather) only for they to be a burst of sunshine. It would be so nice if victron support integration with a remote shunt to allow it control current floor to the battery (this might happen in future firmware upgrades) for now blanket reduction of the rated currents outoput of the controller is not something I want to do.

mcgivor · December 2019

What you could do is use a DC current sensor with a dry contact which drives a DC rated relay, the contacts of which would disable a string of the PV at a pre determined current threshold, thereby limiting charging current when needed. During periods of cloudy weather the full limited potential would be available because the threshold wouldn't be met.

Estragon · December 2019

I really doubt you're going to see "too much" current in bulk. Re-reading several manufacturers charging recommendations, it seems clear to me that C/10 charge capacity is recommended for grid charging, as total charging time exceeds available solar pretty much anywhere on earth. For solar applications, Rolls recommends 10-20% as a minimum pv/controller capacity, and gives no maximum.

The only time I see "high" (>~C/5) charging current is when I'm bulk charging with the genny, and the sun pops out unexpectedly. When this has happened, I haven't seen any material heating of batteries or other ill effects. With roughly similar pv to bank capacity ratio as you, I've never seen "high" current on pv alone. Maybe a mountain top location could, but I don't.

fratermus · December 2019

bigbrovar said:

I am hoping that the system would be past the absorption stage before the panels hit peak generation mostly due to the size of my array (3kw).

My van install is much smaller (570w, 220Ah) but it behaves as you say here. It starts making power so early that current is dropping in Absorption well before solar noon. About the only time I get a "full pull" is when camping in a forest with a hole in the canopy overhead. Even then my flooded GC2 batts don't seem to want much more than C/5.

Flooded Acid Charge Rate

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