# Typical Sealed flooded lead acid internal rsistance ??

Solar Expert Posts: 43
I am getting nowhere with the battery manufacturer; I hae 585 AH of new 12 volt automotive batteries (yes I know I chose wrong but have financial constraints).

Generally my PV output will handle my Air Conditioner Load (550 watts) load maintaining 12.6V or better as of course I only need it when it is hot out. However when a cloud passes overhead the voltage will quickly drop and my LV alarm sound. This is not truly LV as it will quickly rise if I switch of the air. It is in fact the voltage drop caused by the 55 amp AC load due to the internal resistance of the batteries.

I need to figure out an APPARENT safe minimum voltage for the alarm without dropping below 50% battery charge.

Any ideas or your typical internal battery resistances? It would appear to me that at 55 amp load on 585 AH of batteries it MIGHT me as low as 11.9V.

PS my battery wire Voltage drop is insignificantly low.

• Solar Expert Posts: 318 ✭✭✭✭✭✭
Re: Typical Sealed flooded lead acid internal rsistance ??

Battery internal resistance varies with SOC. The lower the SOC, the higher the apparent internal resistance. You can crudely calculate it by using ohms law. R=(E/I) It will be in the milliohms for a good bank.

Here is a link to a page that has a lot of info on cell resistance.

http://www.alber.com/Docs/PredictBatt.pdf
• Solar Expert Posts: 43
Re: Typical Sealed flooded lead acid internal rsistance ??
Joe_B wrote: »
Battery internal resistance varies with SOC. The lower the SOC, the higher the apparent internal resistance. You can crudely calculate it by using ohms law. R=(E/I) It will be in the milliohms for a good bank.

Thanks but in that case I don't see as I need to calculate it? If V drops by say 0.6 volts inside 15 seconds then either I have a completely SHOT set of new batteries or the 0.6 volts (at 55 amps discharge) is the effec t of internal battery resisistance and I should set the LV alarm for less than 12V.

Or am I missing something here?

Pete.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Typical Sealed flooded lead acid internal rsistance ??

pressing automotive batteries into this kind of service they won't last long, but if you think it is just the batteries being drawn down in voltage by the huge load then this will be born out by the at rest voltage of after say 3 or 4 hours as that will show the true battery voltage dropped. from there you can figure for any adjustments you may wish to make on the lvd.
• Banned Posts: 17,615 ✭✭
Re: Typical Sealed flooded lead acid internal rsistance ??

I have to wonder how you know you have 585 Amp hours of battery, as automotive batteries usually do not give specs on Amp hour rating because it is irrelevant to the application.

I also have to wonder where you got the 550 Watt rating for the A/C unit, as that will vary according to how hard the compressor is having to work. Have you checked this with a Kill-A-Watt? The start surge will be much higher too.

Using the given numbers and the 85% efficiency of a typical inverter the DC Amps are indeed likely to be 55, which is nearly 10% of the total bank capacity: the absolute maximum you'd want to draw from them. The internal resistance will not fall linearly either, depending on the Peukart effect particular to those batteries. Auto batteries it's likely to be not good, as they are designed to deliver high Amps briefly, then "recover" a bit and try again. On a steady-state draw they are likely to drop dead very quickly.

Sorry, but using unsuitable components inevitably results in unsatisfactory performance.
For that kind of load I'd want to use a 24 Volt system at least, if not a 48. I know; it's too late now, the money's spent.
• Solar Expert Posts: 318 ✭✭✭✭✭✭
Re: Typical Sealed flooded lead acid internal rsistance ??
SirSparks wrote: »
Thanks but in that case I don't see as I need to calculate it? If V drops by say 0.6 volts inside 15 seconds then either I have a completely SHOT set of new batteries or the 0.6 volts (at 55 amps discharge) is the effec t of internal battery resisistance and I should set the LV alarm for less than 12V.

Or am I missing something here?

Pete.

I assumed from the title of the thread that you were looking for information on how to characterize FLA cell resistance. In any case, any cell will have an internal resistance associated with it. The drop of the pack voltage under a given load is dependent on this resistance (which varies with SOC) Since you have a load that can vary and the SOC of the bank is not known, it is not possible to simply set a threshold and a timeout. If your load was constant then maybe; but without knowing both in real time, its pretty difficult.
• Solar Expert Posts: 43
Re: Typical Sealed flooded lead acid internal rsistance ??

The batteries arer 195AH because that is what they STATE on them and 3 x 195 = 585 !

I don't need start up loads I am more than capable of calculating that as well as run current.

I guess nobody knows the answer so I shall go with the apparent; at 12.6V battery state and 55 amp load will reduce V to about 11.9V until the cloud passes where it will immediately climb again.

Re-wiring a 12V RV with existing 250 amp alternator for 24V is NOT an option.
aqnd in any case distance between PV/CC/battery is exrtremely short.
• Banned Posts: 17,615 ✭✭
Re: Typical Sealed flooded lead acid internal rsistance ??

You were given the answer: the internal resistance of a battery is not fixed nor linear nor are the characteristics universal across all battery makes and models. You can not predict the SOC of a battery based on Voltage and current draw over time because of this.

In essence you are trying to run a massive load on a mere 12 Volts. It is an inadvisable undertaking. There's nothing you can do about the laws of physics. At best those batteries would support the load for about 5 hours of run time. Even if you had enough PV to "max out" a 60 Amp controller your load is nearly as much as it could supply at full power.

Good luck with whatever you settle on for LV point.
• Solar Expert Posts: 43
Re: Typical Sealed flooded lead acid internal rsistance ??
You were given the answer: the internal resistance of a battery is not fixed nor linear nor are the characteristics universal across all battery makes and models. You can not predict the SOC of a battery based on Voltage and current draw over time because of this.

In essence you are trying to run a massive load on a mere 12 Volts. It is an inadvisable undertaking. There's nothing you can do about the laws of physics. At best those batteries would support the load for about 5 hours of run time. Even if you had enough PV to "max out" a 60 Amp controller your load is nearly as much as it could supply at full power.

Good luck with whatever you settle on for LV point.

Thanks but I need 5 minutes NOT 5 hours (until ther cloud passes) Otherwise it will switched.to shore supply anyway. It is also an MXC80 not 60 and reigisters 690 to 790 watts when I need A/C.

I was trying to ascertain the answer to a simple question; What do others see in the way of voltage drop with high (not massive) loads, irrespective of battery type but I am being given the run around too much so withdraw the question.

However it would have been nice to see a question attempted to be answered w/o a lot oif superfluous questions. which make the assumption I am an idiot !
Thanks.

Pete (State Certifified Master Electrician)
• Banned Posts: 17,615 ✭✭
Re: Typical Sealed flooded lead acid internal rsistance ??

Pete;

You have not "been given the run around". You have been given the facts and physics as they pertain to lead-acid batteries as per your original inquiry. It is regrettable that the answer is a simple "X Ohms", but the question is not as simple as it would first appear. In attempting to give you the information you want you were asked civil, not "superfluous", questions in regards to checking the base data you were working with. This is not making the assumption you are an idiot, it is double-checking the facts. Along that line is the inescapable fact you are trying to power a very heavy load with a low Voltage system. That you have an FM80 not an MX60 is neither here nor there; you are dealing with very heavy current which inevitably makes the system inefficient: the more current drawn the more power goes to heat rather than work and the hotter the wires become (even though you can't necessarily feel the heat) resulting in their resistance going up necessitating even more current draw to meet the power demands.

If you were to look at the discharge rate of a true deep cycle battery you'd notice the Voltage fall off a small amount immediately upon having a load put on it. It will then hold a Voltage level under that current draw for a while, then start to drop rapidly down to dead. Automotive batteries perform far worse under the same circumstances. Yes, you may as well guess at a Voltage cut-out point because you don't have enough information to calculate it from (which would have to include the performance curve of the batteries). Sorry it is not simple.

Frankly, your attitude is uncalled for.
• Solar Expert Posts: 43
Re: Typical Sealed flooded lead acid internal rsistance ??
You were given the answer: the internal resistance of a battery is not fixed nor linear nor are the characteristics universal across all battery makes and models. You can not predict the SOC of a battery based on Voltage and current draw over time because of this.

In essence you are trying to run a massive load on a mere 12 Volts. It is an inadvisable undertaking. There's nothing you can do about the laws of physics. At best those batteries would support the load for about 5 hours of run time. Even if you had enough PV to "max out" a 60 Amp controller your load is nearly as much as it could supply at full power.

Good luck with whatever you settle on for LV point.

I was trying to ascertain the answer to a simple question; What do others see in the way of voltage drop with high (not massive) loads, irrespective of battery type

For example this would have bveen nice;

I have an X amp (type) battery when I connect Y amps I see an immediate drop of Z volts.` I can extraspolate the rest,.

Never mind !
• Solar Expert Posts: 630 ✭✭
Re: Typical Sealed flooded lead acid internal rsistance ??

Dear Pete,

I took a Costco marine battery(75Ah) X
Connected to 1500 Watt HFT Inverter. Y
Tried to run a 10000 btu A/C. Z

Everything was connected via #4 battery cable, longest was 18 inches.

It ran 5 minutes before the Amps spiraled upward out of control, and blew a 80amp fuse.

EDIT: I had a DC clamp meter on the battery during this test, and the 80 Amp fuse didn't actually blow until the clamp meter read well over 120 Amps. As a result I switched to 40 A fuse in the positive lead of the battery to the inverter.

I knew from the start it wouldn't run long, but I needed a proof-of-concept as a DIY. Thanks to a Fuse, I didn't have anything expensive to replace and the power pack runs all my light-duty needs just fine.
• Solar Expert Posts: 43
Re: Typical Sealed flooded lead acid internal rsistance ??
bmet wrote: »
Dear Pete,

I took a Costco marine battery(75Ah) X
Connected to 1500 Watt HFT Inverter. Y
Tried to run a 10000 btu A/C. Z

Everything was connected via #4 battery cable, longest was 18 inches.

It ran 5 minutes before the Amps spiraled upward out of control, and blew a 80amp fuse.

I knew from the start it wouldn't run long, but I needed a proof-of-concept as a DIY. Thanks to a Fuse, I didn't have anything expensive to replace and the power pack runs all my light-duty needs just fine.

Thank you Bmet. I consider that the first positive reponse I have had. Albeit my bank is 585AH and my A/C is only 5,000 BTU.

Excuse me but I just got annoyed with some people presuming I knew nothing and asking silly questions which pertained in no way to my original query.

If a person states his battery bank is 585AH thern HECK just accept it !!!
Same with the current (power) draw of 550 watts; JUST Accept it.

Perhaps it's time I moved away fromn beginers quarterrs ?

PS your A/C likely stalled and then caused to spiralling amps
• Banned Posts: 17,615 ✭✭
Re: Typical Sealed flooded lead acid internal rsistance ??

A car battery is not a deep cycle; it will not perform the same.
An AGM is not an FLA; it will not perform the same.
A Trojan is not a Crown; it will not perform the same.
As far as extrapolating from someone else's experience with a completely different system, you might as well guess.

Or you could disconnect the PV, run the A/C until the battery reads 12 Volts (approximating 50% SOC), turn off the A/C and check the Voltage "rebound". That difference would be the maximum below 12 VDC you could allow the system to go before shutting down the A/C load.

That's all.
• Solar Expert Posts: 43
Re: Typical Sealed flooded lead acid internal rsistance ??
A car battery is not a deep cycle; it will not perform the same.
An AGM is not an FLA; it will not perform the same.
A Trojan is not a Crown; it will not perform the same.
As far as extrapolating from someone else's experience with a completely different system, you might as well guess.

Or you could disconnect the PV, run the A/C until the battery reads 12 Volts (approximating 50% SOC), turn off the A/C and check the Voltage "rebound". That difference would be the maximum below 12 VDC you could allow the system to go before shutting down the A/C load.

That's all.

OOPS ! I misread;;;;;;; OK I will try that thanks.
This discussion has been closed.