Charge rate in a serial string

Dusty

I'm using 4, 12v AGM batteries in series to make a 48v string (with two strings in parallel, so 8 batteries total). If a single AGM battery is rated by the mfr to accept 12v 10A charge current, since I have four in series, am I still limited to 10A to charge that 4 battery string?

I know that since I have a parallel bank of two strings, I can charge the bank with 20A and not exceed the mfr's recommended charge rate, but I wasn't sure if charging current changed because of having 4 batteries serially in each string.  Since Ah capacity doesn't change with batteries in series (but Kw capacity does), my gut tells me that I'm still limited to 10A charge current per string.

I've configured my XW6048's internal charger to only push 25% of its capability from the grid to my battery bank (but I usually keep it disabled anyway), based on my gut of 20A total charge current allowed to my batteries, but deciding what to adjust my SCC to--or if I need to at all--is a bit harder.  Since my batteries are usually already topped off and in Float by the time the PV starts making significant harvest, I'm not too worried about day-to-day operation.  But if the grid is down and my batteries are depleted, when the sun comes out, my PV is capable of providing about 45A to my battery bank, which would be more than double the mfr recommended rate--if my gut feeling about the 10A limit per string is valid.

Estragon

Your gut is right. In series, amp-hour capacity stays the same, so the max charge rate (being defined as a fraction of ah capacity) does as well. The wattage is higher, but current (amps) is the same.

In parallel, ah capacity does increase, so charge rate should also increase.

Even without an extended outage, the controller could theoretically put high current to the batteries for a time going from resting or float voltage to absorb. Probably best to limit the controller to recommended max just in case.

mcgivor

Your gut is telling you correctly, 20A, 10A per string, but that is the recommended current, you could actually go a little higher, then comes the question of loads, if the current is limited from the array say at 20A but the loads draw 30A, for example, there would be a net deficit of 10A which would be drawn from the battery. For this reason it's important to know what the average loads are in order to program the limitations to include the loads. In the example above the limit would be 50A, 20A for the battery 30A for the loads. It dose however get tricky when loads are intermittent, constant loads are easily calculated .A clamp on ammeter with maximum, minimum and average capabilities would be helpful to establish a base line for an average, but depending on how large the loads are, it may be ok to hit the batteries with a higher current, as long as it's for short durations, as the batteries will only accept what they can absorb, so if in a full state of charge, current absorbed would be minimal.

Dusty

Thank you for validating my gut @estragon. And more great points to consider regarding concurrent loads while charging @mcgivor!

Since my vital loads are the intermittent compressors of my refrigerator and chest freezers and very little else in the way of constant loads (I'm thinking about being forced off-grid due to a hurricane--'tis the season!), I've used a kill-a-watt meter to determine daily kwh consumption for everything I might run but I don't have a way of determining surge current when the compressors first start, since it's not on the nameplate data.  Although I'd like to be able to figure that in, since it's only for a second or two, I'm hoping that it wouldn't be long enough to stress the battery bank from the SCC over-charging for that brief period of time. The units draw approximately 120W each when running, so I'll say (non-surge) a total of 7.5 amps at 48v.  Since my max observed PV output has only been about 42A, subtracting 20A max charge current, I think I'd be safe if I set my SCC to 40% max charge rate (24A).   

But that's only when I'm in a grid-down scenario.  99.9% of the time, I'm grid-tied and in sell mode, so the amperage leaving the battery bank is being concurrently supplied by the PV.  This is where it gets "sticky," because at first glance you mind think it's a zero sum issue--whatever comes into the battery by PV is immediately used by the loads and then the excess is sent back to the grid.  But there's that darn DC ripple effect on the battery bank.... 

Thanks again!

Dusty

Here's the datasheet charging specs on the AGM battery I'm using:

"Recommended Charging Information:
Alternator: 13.65 to 15.0 volts
Battery Charger (Constant Voltage): 13.8 to 15.0 volts; 10 amps maximum; 6-12 hours approximate
Float Charge: 13.2 to 13.8 volts; 1 amp maximum; (indefinite time at lower voltages)
Rapid Recharge: Maximum voltage 15.6 volts. No current limit as long as battery
(Constant voltage charger) temperature remains below 125°F (51.7°C). Charge until
current drops below 1 amp.
Cyclic or Series String Applications: 14.7 volts. No current limit as long as battery temperature
remains below 125°F (51.7°C). When current falls below 1 amp,
finish with 2 amp constant current for 1 hour.
All limits must be strictly adhered to."

But it's interesting to see that they say for series string applications, there's no current limit as long as the batteries are under 125 deg F.

Estragon

Assuming you're using a remote temp sensor, the controller should dial back absorb voltage at increased temps.

The charger max appears to apply for a charger with only a single fixed voltage. If I'm reading it right, the last paragraph ("cyclic...") is more applicable in your application and controller capabilities.

Dusty

I agree with you.  And based on that, there doesn't appear to be a current limit.

I do have a temperature sensor, and I've never experienced an increase in temperature, even with the SCC set to 100% charge rate.

Estragon

AGMs are pretty efficient. Unless in death valley in summer, 125°f would be telling me something is seriously wrong.

BB.

AGMs are very efficient--Until they approach the end of their life (or are abused, such as charging with too high of absorb/float voltage).

If an AGM battery/bank gets hot--You have something to worry about.

-Bill