Battery voltage drops with high power

lazza

HI Forum

Well it's been a long time- I've been too busy to even enter the forum. But love the new look! Congratulations on the upgrade of the forum, looks great.

I have a quick question about battery voltage drops and battery life.

We've seen an installation where a guy has 4 x 250Ah 12V normal flooded lead acid batteries in a 24V system.
His pump is ludicrously inefficient to irrigate about 4 cabages and is pulling 100A from the batteries- that is to say about 2.5kW. (the inverter is 3kW so can manage)

However, as expected, the voltage drop is very high, dropping to about 23.5V from full batteries and within 20 mins dropping to around 22.4V.

My question is the following:

-Is it very damaging to bring a 24V system down to say 22V under high power conditions due to the voltage drop ??

-Or is it much less damaging than slowly bringing the batteries down to 22V through many hours of low power consumption??

Thanks for any assistance

Cheers
Larry

Blackcherry04

You stated the problem, The Discharge rate is to high for the batteries you have. Assume you have sufficient cabling, the voltage sagging with a load of more than 1.5 - 2% discharge rate on flooded batteries is to be expected. @ 22v the Inverter is in a major stress. Below are some suggestions that can be used to mitigate the sag.

1) Increase the system voltage.

2) Increase the battery bank size.

3) Increase the battery cabling size.

3) Switch to AGM or Lithium Batteries that have a higher discharge rate.

4) Get a generator.

CALLD

I've seen those exact figures (nearly) with a very similar resistive load (2400w boiler) on my 24v system when I had a 300Ah flooded battery bank. I've now moved to 260Ah AGM batteries and with a similar load I see 24.6v initially dropping down to 23.6v after about 30mins.

I don't think it's bad until it gets below 22v with such a heavy load. Remove the load and see what the voltage rebounds to after about 2 hours. I often find the rebound voltage after a few hours is around 25volts which means the batteries are still ok.

But as many people will tell you if you want to run heavy loads you should either get a bigger battery or go with AGM. Batteries are happiest if the loads they are expected to support do not ever pull the voltage under 2vpc (24v for a 24v system).

westbranch

I addition he could add more PV to add to the battery power being used, but still needs to go to 48v as BC04 said...

Sam I am

Battery voltage drops with any load applied.

We installed an A/C coupled system with the Magnum MS4448PAE inverter and a 48 v 250 Ah NiFe battery bank. In February, an ice storm hit our area and we lost power for three days. When we switched to battery power the voltage dropped from 52.7 to 36 (drop out setting) in about 12 minutes. The state of charge on the inverter display remained at 98% while this voltage drop took place. Finally last week the company that installed the system came out to diagnose the problem. According to "Be Utility Free" the company that supplied the batteries said the charger must generate 68.2 v to completely charge the batteries and Magnum says the inverter will only go as high as 64v. Also it is my understanding that NiFe batteries can go down to near zero without damaging the batteries. Magnum also says that 36v is as low as the inverter will go as far as the "drop out" voltage is concerned.
Needless to say we spent a lot of money on this system and ask the question several time as to the compatibility of the inverter and NiFe batteries. Are these two compatible?
​We know very little about solar systems so keep that in mind with any rsponses.

vtmaps

Sam I am wrote: »

Battery voltage drops with any load applied.
We installed an A/C coupled system with the Magnum MS4448PAE inverter and a 48 v 250 Ah NiFe battery ban

Welcome to the forum, Sam.

I suggest you start your own thread and make sure "NiFe" is in the title. That will catch the eye of some of the forum members who use NiFe. Have you searched this forum for "NiFe"?

--vtMaps

inetdog

vtmaps wrote: »

Welcome to the forum, Sam.

I suggest you start your own thread and make sure "NiFe" is in the title. That will catch the eye of some of the forum members who use NiFe. Have you searched this forum for "NiFe"?

--vtMaps

And instead of using the built-in forum search, you may get better results by using Google and adding site:forum.solar-electric.com to your search string.

CALLD

It must also be noted that some batteries cannot hold a high rate of discharge without "rest periods". This has got to do with the rate at which the acid concentration gets depleted at the plates. Batteries with sponge lead plates can provide brief high current surges of maybe a few minutes and then require time for the acid to diffuse back into the plates before the next high current surge can be provided.
Deep cycle batteries that use smooth plates do not suffer so much from the depleted acid effect and can hold the load for a longer period.
AGM batteries have very tightly spaced plates resulting in a much more even concentration of acid spread.
"A 100Ah flooded lead acid battery with sponge lead plates could give you no more than 25Ah of usuable power under a 100amp load". That's no more than 15mins run-time under a C/1 load!

inetdog

CALLD wrote: »

"A 100Ah flooded lead acid battery with sponge lead plates could give you no more than 25Ah of usuable power under a 100amp load". That's no more than 15mins run-time under a C/1 load!

Let's hear it for Peukert's Law!