Maximum Battery Load Current

Coach Dad · June 2012

I’ve been running my off-grid PV system for almost 2 months now and it is working nicely. I’m running four 6 volt 305 AH AMG batteries in series.

I was watching the display of the inverter Saturday night while my wife had the vacuum cleaner going and saw that it was drawing 45 DC amps and loading the voltage down (I don't recall the exact voltage).… But the State of Charge was at 91% after she was done. (well within the 50% target) The next morning the State of Charge was at 88% and was back to 100% shortly after the sun came out.

But seeing the current at 45 amps got me thinking.…
We are always talk about not draining the batteries below 50% SOC… but I never see any discussions regarding the maximum load current.
I see charts on the battery web site that list 1hr, 2hr, 4hr rates all the way up to 120hr rates but I don’t see anything which says don’t exceed a specific number of amps while discharging the battery.
I went through the battery manufacturers manual and it lists a maximum charge current of 5C (500A for a 100AH battery) but it doesn’t list the maximum discharge current.

So my question is… I there a “maximum discharge current” rule of thumb, or is the rule, anything goes as long as you stay within 50% DOD?

P.S… I did ask her to avoid vacuuming at night.

Cariboocoot · June 2012

Re: Maximum Battery Load Current

Well you're quite right we don't talk about it much. Maybe we need a section for "Infrequently Asked Questions".

As a rule, the discharge rate should not exceed the charge rate. In other words you'd have a C/8 rate either way. But just as you can charge at higher than normal rates you can discharge at higher than normal rates. Not for long, because the faster you draw current off the lower the effective Amp hour capacity of the battery. And the old rule of higher current = more heat is in play of course, and also not exceeding the maximum the wiring and circuit protection can handle. So if you set up your system to handle the inverter's full output the only effect of the high discharge rate is the rapid depletion of the batteries.

This is one of the thing we warn people about in a round-about way; that higher current draws can "flatten" a smaller battery bank and cause an immediate Voltage drop that can sometimes shut the inverter down.

Heck, many people have discharged whole batteries in an instant. The sparks and flames are quite spectacular!

BB. · June 2012

Re: Maximum Battery Load Current

You need to look a the battery specifications... But for a flooded cell battery bank it would run from C/8 for max continuous load (running for hours at a time) to around C/2.5 for starting surge (exceeding this value may "collapse" the battery output voltage--especially if the bank is not fully charged).

The more you exceed C/8, the less "apparent capacity" your battery bank will have (it will discharge even faster).

For AGM type cells, they can even take upwards of C*4 -- Or complete discharge in ~15 minutes (used very often for UPS applications).

-Bill

Coach Dad · June 2012

Re: Maximum Battery Load Current

So if I understand you correctly...
The rule of thumb is... Don't exceed your charge rate AND keep the DOD above 50% AND keep within the limits of wiring/inverter.

So using my AGM battery bank as an example. The max charge rate is 5C (5x305=1525Amps). Since that is an unrealistic number, the only limit is the 4kw inverter and make sure I don't go below 50% state of charge...

No negative effects on battery life???

Cariboocoot · June 2012

Re: Maximum Battery Load Current

Always negative effects on battery life. There's almost nothing you can do with a battery that doesn't have a negative effect on its life!

An occasional "hard hit" won't make much difference. But high current dragging it down fast over and over ... that's why there are those different Amp hour capacity ratings for different discharge times.

BB. · June 2012

Re: Maximum Battery Load Current

Anytime you "stress" a battery/battery bank, you "age" it... So keeping loads in the C/8 range is better, but if you on occasion do a C/5 load, it is not going to kill the bank tomorrow.

Similar for 50% maximum discharge--This is the recommendation for normal discharging. If you always cycle below this number, you will have a limit on the cycle life--But even then, you may have other life limiting events (aging, accidental 100% discharge) which will eventually end the life of your battery bank.

Several things to "avoid in general":

Avoid discharging below 20% state of charge ever--You run the chance of killing one or more cells in your battery bank.
Avoid over charging (too high of charging voltage). AGM's can out-gas electrolyte/gases--which cannot be replaced (unlike flooded cells).
Avoid elevated equalizing voltage on AGM cells (see above).
Avoid letting batteries set for days/weeks/months below ~75% state of charge. Batteries sulfate faster when less than full charged. Recharge unused flooded cell batteries once every month. Recharge AGM once every 3 months or so.
Avoid running batteries "hot"... Roughly, for every 10C (18F) above room temperature, the battery will lose ~1/2 its life.
Keep cables/connections clean.
Monitor cell/battery/bank voltages looking for "something different". It is possible for a "bad cell" to take out an entire series string of batteries (if open cell) or an entire bank (if shorted cell).
For parallel connected batteries, I would suggest a DC Current Clamp meter so you can monitor the charging/discharging current in parallel battery strings (identify bad connections, week cells, etc.).
I like Battery Monitors (Victron is another good brand), especially for AGM/sealed battery banks where you cannot measure specific gravity.

-Bill

Coach Dad · June 2012

Re: Maximum Battery Load Current

Thanks guys... I am glad I asked.

What about hitting them while the sun is shining and the batteries are in Absorb mode? Can the PV output be factored in?

For example if my PV's (1410watts) and Charge controller have the ability to produce 60 amps (call it 40 Amps after all the losses)...
Lets say C/8 for my system is 305/8 = 38Amps...
Lets say the sun is shining and my PV's were able to produce 40 amps out of the MPPT60 (1410 watts of PV)
If my vacuum is using 45 amps, are the 40 amps out of the MPPT and 5 from the battery hitting the inverter or is it more complicated than that?
Does using the above logic allow me to safely run 78 amps (38 +40 = 78 )?

2manytoyz · June 2012

Re: Maximum Battery Load Current

What I found interesting is Trojan says the following about their batteries: "Trojan’s batteries take between 50 – 100 cycles to work up to providing full, peak capacity"

Reference page 18: http://www.trojanbattery.com/pdf/TRJN0109_UsersGuide.pdf

Not disputing hard charging/discharging will shorten the battery life, but looks like they need to be exercised initially to get them up to par.

Cariboocoot · June 2012

Re: Maximum Battery Load Current

Coach Dad wrote: »

Thanks guys... I am glad I asked.

What about hitting them while the sun is shining and the batteries are in Absorb mode? Can the PV output be factored in?

For example if my PV's (1410watts) and Charge controller have the ability to produce 60 amps (call it 40 Amps after all the losses)...
Lets say C/8 for my system is 305/8 = 38Amps...
Lets say the sun is shining and my PV's were able to produce 40 amps out of the MPPT60 (1410 watts of PV)
If my vacuum is using 45 amps, are the 40 amps out of the MPPT and 5 from the battery hitting the inverter or is it more complicated than that?
Does using the above logic allow me to safely run 78 amps (38 +40 = 78 )?

Yes. This is the "load shifting" you may have seen mentioned here before. Taking advantage of the power potential of the panels after the batteries have had all they can take. It works fine.

Jim45D · June 2012

Re: Maximum Battery Load Current

Here's a nice little bit of info I got from a tech at Concorde when inquiring about my battery's state. It went something like this: "Not to worry, here's the dirty little secret about deep-cycle batteries. They take 30 to 60 charge/discharge cycles to reach their full capacity. Those new deep cycle batteries are not working like you thought they should, and they won't just yet, give 'em a little time. Rest assured, we refer to it as a "Break-In-Cycle". They will get better." That was almost 7 years ago, and he was right.

BB. · June 2012

Re: Maximum Battery Load Current

And, yes, the charge controller will attempt to keep the battery at Vbatt-charge voltage--even if some (or all) of the current is used by your loads.

Obviously, if you use 40+ amps from 9am to 3pm, your batteries will never really get a proper charge (they need current flowing into the batteries to charge--A standard charge controller cannot tell if it is recharging battery banks or if some/all of the current is being diverted to loads).

-Bill

mike95490 · June 2012

Re: Maximum Battery Load Current

Coach Dad wrote: »

Thanks guys... I am glad I asked.

What about hitting them while the sun is shining and the batteries are in Absorb mode? Can the PV output be factored in?

For example if my PV's (1410watts) and Charge controller have the ability to produce 60 amps (call it 40 Amps after all the losses)...
Lets say C/8 for my system is 305/8 = 38Amps...
Lets say the sun is shining and my PV's were able to produce 40 amps out of the MPPT60 (1410 watts of PV)
If my vacuum is using 45 amps, are the 40 amps out of the MPPT and 5 from the battery hitting the inverter or is it more complicated than that?
Does using the above logic allow me to safely run 78 amps (38 +40 = 78 )?

That's what the timer on my pump is for, lets the batteries charge till noon, then kicks the pump on, and hopefuly, I'm running 1KW motor off the sun via a 3KW array. Any surplus continues to top off the batteries.

frankson · June 2012

Re: Maximum Battery Load Current

Jim45D wrote: »

Here's a nice little bit of info I got from a tech at Concorde when inquiring about my battery's state. It went something like this: "Not to worry, here's the dirty little secret about deep-cycle batteries. They take 30 to 60 charge/discharge cycles to reach their full capacity. Those new deep cycle batteries are not working like you thought they should, and they won't just yet, give 'em a little time. Rest assured, we refer to it as a "Break-In-Cycle". They will get better."

I'd heard something similar before, thanks for reminding me of it here.
So, question now is; How deep a discharge is required to count as one of the initial break-in 30 to 60 cycles?
50%, 5%, or something in between?

BB. · June 2012

Re: Maximum Battery Load Current

I would tend towards 25% or heavier discharges... 5% is not going to do it (and is recommended to "deeper cycle" a storage battery that normally only has 5% cycling once in a while--from what I have read).

-Bill

65DegN · June 2012

Re: Maximum Battery Load Current

Coach Dad wrote: »

So using my AGM battery bank as an example. The max charge rate is 5C (5x305=1525Amps).

Charging or discharging at a rate of 1,525 amps would destroy those batteries in short order, probably minutes.
C5 refers 1/5 the overall battery in amp hours (division not multiplication) . So a C5 rate for a 305 AH battery would be 61 amps. At 24 VDC thats 1,464 watts.

I know of a system that was abused with a load of ~ 20 KW on a bank of new Surrettes that had an AH capacity of around 50 KWH. It ruined them in an evening. I suspect it melted the plates because capacity dropped to just a couple AH. Several of the 2 V batteries now drop to 0 V with a relatively light load within 5 minutes.

Coach Dad · June 2012

Re: Maximum Battery Load Current

65DegN...
The AGM batteries allow a 5C recharge rate.. I've highlited the section out of the maunal.. They even included an example.
Attachment not found.
However the wires would fry at 1500 AMPs if you could find something that could charge at that rate...

Blackcherry04 · June 2012

Re: Maximum Battery Load Current

Coach Dad wrote: »

65DegN...

However the wires would fry at 1500 AMPs if you could find something that could charge at that rate...

Not to mention. it'd be a heck of a fuse to protect it.

I think their numbers are all screwed up.

Jim45D · June 2012

Re: Maximum Battery Load Current

frankson wrote: »

I'd heard something similar before, thanks for reminding me of it here.
So, question now is; How deep a discharge is required to count as one of the initial break-in 30 to 60 cycles?
50%, 5%, or something in between?

I ask the same question, and his reply was...."never go below 12.20, and try to stay around 12.30." According to my graphic card 12.20 is 55% DOD, and I strive to always stay in the area of never below 12.40, 12.25 is 50% DOD.

frankson · June 2012

Re: Maximum Battery Load Current

Coach Dad wrote: »

65DegN...
The AGM batteries allow a 5C recharge rate.. I've highlited the section out of the maunal.. They even included an example.
Attachment not found.

I've got 4 Lifeline AGM 8D's for total of 1,020 amps, which, according to above, I should be charging at a little over 200 amps.

Best I usually see with solar charging alone here, without getting generator involved, is 80 amps, but that seems to work OK
based on resting voltage next morning (after 12 hr rest of no charging/load) of 13.05 VDC. Of course, I rarely discharge below
80% SOC, about 200 amps to replenish, before fully re-charging.

Anybody see a problem with what I'm doing above, in light of that AGM manual claiming negative effects on battery cycle life
if charging current is less than their proscribed 20 amps current per each 100 Ah of battery?

BB. · June 2012

Re: Maximum Battery Load Current

And here is where it gets "complicated" Surge vs Sustained loading/charging.

Surges should be just for a few seconds, where sustained loads is for hours (8 hours is a common value).

So, C*5 will only last for ~12 minutes (or less)--Small UPS systems are designed (spec'ed) to discharge in 15-30 minutes (C*4 to C*2), but I would not expect the battery pack to last much more than 1-2 years (the small UPS's probably do not have have the best charging/floating circuit designs--i.e., low cost is first and foremost).

Electric Wheel Chair Batteries--They probably replace those every ~2 years. One person reported that some vendors have about a 20% early life failure with wheel chair batteries... So, do not be surprised if you end up with a defective battery somewhere.

Also, one other warning--Many of the "higher end" smart MPPT (and PWM) charge controllers operate from Battery Power--Leaving them connected to a solar array while connecting/disconnecting the battery bank may cause the controller some "confusion".

In general, they really do operate better if they are always connected to a battery bank.

Another suggestion for the charger battery bank + inverter... As you get other "electric" devices, they may have other voltages than 36VDC, and they may have other battery chemistries. Using a Battery Charging station + Inverter may make the charging station a bit more adaptable. People bring their own 120 VAC battery charger (scooters, bikes, even cell phone) and they can plug into the generic 120 VAC outlet (with in reason).

-Bill

niel · June 2012

Re: Maximum Battery Load Current

i never charged at a .2c charge rate for my sunxtenders. i think the best i did/do is near 13%, but most times it's lower (near 5%-6%) as that higher rate represented solar + an ac charger. i had a sunxtender for 8yrs before it crapped out on me and i did give it some abuse at times. (vented a few times + high and deep discharges on a few occasions) i still have it in reserve as it will kick a few ah out yet. i can't say it might've done better with .2c, but i'm happy with how it did.

Coach Dad · June 2012

Re: Maximum Battery Load Current

niel wrote: »

...
i had a sunxtender for 8yrs before it crapped out on me and i did give it some abuse at times. (vented a few times + high and deep discharges on a few occasions) .

Niel - what kind of DIScharge currents were you pulling?
I'm still trying to figure out if drawing 40-60 amps for 10-20 min at a time is ok as long as the SOC (State of charge) stays above 80%.

vtmaps · June 2012

Re: Maximum Battery Load Current

Coach Dad wrote: »

I'm still trying to figure out if drawing 40-60 amps for 10-20 min at a time is ok as long as the SOC (State of charge) stays above 80%.

yes, but. How are you determining that your SOC remains above 80? If you are using a battery monitor, does it account for Peukert effect? In other words, most battery monitors see 1000 watts for a minute the same way they see 50 watts for 20 minutes. In reality the 1000 watts for a minute load lowers your SOC more than the 50 watts for 20 minutes load.

One other thing to watch for when drawing large loads is the voltage. Don't let it get too low, regardless of the SOC.

--vtMaps

niel · June 2012

Re: Maximum Battery Load Current

10-20 minutes would probably be ok to do, but do understand that the curve you are on to determine the soc has changed with the higher discharge rates.

see the charts from windsun for your battery.
http://www.solar-electric.com/cosuagmba.html
now at the 24hr rate this is a draw of 305/24=12.71a per hour. of course you wouldn't do this for 24hrs and you'd stop midway to keep 50% dod. for 80% soc (.2 x 24) you would stop at 4.8hrs at this rate.
now the 8hr rate is 280ah. 280/8=35a per hour drawn. at 50% dod this would stop at 4hrs, but to keep with 80% soc this is .2 x 8 = 1.6hrs. now this is close to the 40a draw you state, but at a 60a draw you are probably in about a 4hr capacity range and the chart does not list it. but i think you are getting my point that you don't have quite as much capacity there as you might have thought with high draws on current so be careful here.

niel · June 2012

Re: Maximum Battery Load Current

i forgot to add that my loads were somewhat variable, but up to about 25a on the 8yr old pvx1040t. needless to say this wasn't good for too long at the top rate, but my savings grace was that i was usually at about 5a 75% of the time.

Maximum Battery Load Current

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