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Maximum output current for battery 4

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eromlignod

eromlignod

Mechanical
Jul 28, 2006
402
Hi guys:

I have an application where I would like to provide low-voltage/high-current power from deep cycle lead-acid batteries.

I understand the concept of "amp-hours" and what they mean to the life of my battery charge. What I don't seem to be able to determine from the battery literature is how much current the battery can safely supply.

For example, I know that a 50 A-h battery can basically provide 25 amps for about two hours. But what if I want 50 amps for one hour...or 100 amps for a half-hour? What is the limit as far as how much current I can pull without substantially reducing the voltage, or causing other problems?

Thanks for any replies.

Don
Kansas City
 
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You must look at the battery makers data sheets as those numbers are all highly dependant on the battery's construction details. Those are all very import questions you've put forth and so are all included in the typical data sheet.

The amount of current you can draw is based on the wire sizes inside the battery and resultant internal heating and the voltage drop caused by the internal wire size, connections, and the plate efficiency (chemical/plate effectiveness).



Keith Cress
Flamin Systems, Inc.-
 
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I see. That's interesting. So should the "1.5 hour rate discharge" current of 3.5 A be considered the maximum reasonable operating current?

Don
Kansas City
 
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"...don't seem to be able to determine from the battery literature is how much current the battery can safely supply. "

Ultra high current applications tend to be the domain of
normal (non-deep cycle) car batteries. Taking it to extremes, some are rated at (for example) "1000 Cold Cranking Amps". This just shows that the upper limit for maximum current from lead acid batteries can be pretty high, provided that the duty cycle and rest time is kept within limits.

UPS applications are also often pretty high current. A typical discharge time might be 20 or 30 minutes, so you can guess that they're being discharged at a fairly high rate.

As Mr. Smoked indicates, the data sheets are supposed to provide this sort of information.
 
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It really depends on the effective series resistance (ESR) of the battery. A typical 1800 mAh AA battery can dump about 10 A for about 10-15 minutes.


The main issue is whether you intend this to be a chronic occurrence. Heavy discharge can alter the internal chemical balance of the battery.


TTFN



 
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eromlignod; Yes that's correct. ~3.5A

I have a product that uses that battery. I draw up to 10A from the battery regularly, but only for about 30 seconds then it drops to only 3s out of 60s. Because I draw so much I have to deal with a larger voltage drop/terminal voltage and a resultant loss of available AHrs instead of 7.2AHr I only get about 5AHrs.

Keith Cress
Flamin Systems, Inc.-
 
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You probably do not really want to specify a deep cycle battery for that type of application. What they call engine starting batteries would probably be more appropriate for extremely high short duration discharge.

Industry practice is to rate batteries at the universal ten hour discharge rate, so a 50Ah battery would be rated to nominaly supply five amps to the end of discharge voltage. You will never get 100 Amps for half an hour from a 50Ah battery, a few minutes perhaps on a good day. But a 50Ah battery will easily supply 500mA for considerably longer than 100 hours.

What happens is that the internal resistance of the battery rises as it is discharged. If you are only pulling an amp or two, a few extra milliohms per cell increase in internal impedance is insignificant. But if you are drawing 500 Amps, a few milliohms rise in impedance per cell can reduce the effective terminal voltage down to just about zero rather rapidly.

How much current can be safely drawn depends on the internal construction of the battery, that is, available plate area and the bonding and current carrying capacity of the internal parts. Severe duty is always going to reduce battery life, and increase the potential for random failures. Warped and buckled plates and open circuits are not exactly unknown at unreasonably high discharge rates.

Try to define your needs, and let a reputable battery supplier recommend an appropriate battery.

Engine starting batteries are worth investigating for really extra severe duty. Fork lift batteries are also another source that are built to survive short burst cyclic high discharge. Things like UPS batteries are better suited to more constant discharge rates over many hours, without the massive peak surge current capability.
 
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Eromlignod:
You said low voltage/high current -- Pse give numbers.

How much current? voltage? for how long ? how many times
per battery charge?




Plesae read FAQ240-1032
My WEB: <
 
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Well, that's what I'm trying to work out. I have a large number of similar devices to run concurrently at 12 V. They take 45 watts each. I was just wondering how many devices I can run with each battery.

The time I can run them is a sort of tradeoff in my situation. Shorter run time might be worth it if I can reduce my cost with fewer batteries driving more devices.

Don
Kansas City
 
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Don:
Are you testing them? why not use line voltage ?
If not test, why do U operate from battery?
If test, can't you reduce the test time?

There are many ways to skin a cat but sometimes
you may find it to be a horse of different color ...

Plesae read FAQ240-1032
My WEB: <
 
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Is this a one-time use battery? If not, then you must run at lower discharge rates, e.g., around 10% of ampacity. Higher discharge rates will degrade the life of the rechargeable battery.

TTFN



 
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"...10% of ampacity. Higher discharge rates will degrade the life of the rechargeable battery."

That's relative. Car batteries are constantly being abused in every possible manner (including being asked to supply up to many hundreds of amps) and they typically last 8 years.

 
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But, even with a car battery, you only pull 300A for a few seconds, which amounts to less than 1% of ampacity.

If you try to pull 300A continuously for 30 minutes, do you think that the battery will not be degraded?

TTFN



 
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If I tried to pull 300A for 30 minutes, I think the battery will be dead ;)

Without numbers, this goes nowhere, so let's see what Don has in mind...


Dan - Owner
Footwell%20Animation%20Tiny.gif
 
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IR: I understood that you were advising to keep the current draw ("discharge rate") to no more than 10% of the amp-hour rating number ("ampacity"). This is often called '0.1 C' (at least for charging cycles) in battery language. In other words, a roughly ten-hour discharge cycle.

Is this what you meant? That was what I was answering.

If I misinterpreted - sorry. Please clarify.

 
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Don, the bottom line really is that a steady power draw off extended over a long period is going to be far kinder to the battery than massive loads of short duration.

Anything is possible, but the size and cost of the battery and power distribution system will increase out of all proportion, if you design around very high peak discharge rates.

Raising your system voltage,(if possible) will reduce the effects of voltage drops and will be more efficient in every way.
 
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I was.

My point was that while you can, and others do, draw large fractions of C on a continual basis, it degrades the chemistry of the battery. Many RC batteries are forcibly discharged at high currents, e.g., 10 amps for 10 minutes from a 2.4 Ah battery, but the batteries lose their rechargeability very rapidly.

On the other hand, a car battery can dump 300A in very short bursts for the entire duration of its life, e.g., 2 times a day for 8 yrs, but the net discharge during each start is only about 0.01C.

TTFN



 
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If Don wants good clear advice beyond what our general teaching has been he should descibe what his loads are, what their duty cycles need to be, the desired lifetime of the battery system, the charging method, and battery environment.

Keith Cress
Flamin Systems, Inc.-
 
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How do you get your car battery to last 8 years.?
Mine only last 3 to 4 years.
The following link is a battery that is often used in computer-sized UPS systems. It is the same size as the usual firealarm system battery of 12v7.2AH but stands high current draws better.
regards, Ray.
 
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