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Maximum output current for battery
4

Maximum output current for battery

Maximum output current for battery

(OP)
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

RE: Maximum output current for battery

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.- http://www.flaminsystems.com

RE: Maximum output current for battery

(OP)
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

RE: Maximum output current for battery

"...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.

RE: Maximum output current for battery

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



RE: Maximum output current for battery

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.- http://www.flaminsystems.com

RE: Maximum output current for battery

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.

RE: Maximum output current for battery

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: <http://geocities.com/nbucska/>

RE: Maximum output current for battery

(OP)
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

RE: Maximum output current for battery

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: <http://geocities.com/nbucska/>

RE: Maximum output current for battery

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



RE: Maximum output current for battery

"...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.

RE: Maximum output current for battery

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



RE: Maximum output current for battery

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
http://www.Hi-TecDesigns.com

RE: Maximum output current for battery

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.

RE: Maximum output current for battery

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.

RE: Maximum output current for battery

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



RE: Maximum output current for battery

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.- http://www.flaminsystems.com

RE: Maximum output current for battery

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.
http://www.gsbattery.com/sla/pe12v9/pe12v9.html
regards, Ray.

RE: Maximum output current for battery

IR: Your "0.01C" example is actually 'average C' (as opposed to 'C').

Given that we agree that 'C' means discharge rate (amps) over ampacity (amp-hrs), then a normal car start might be something like about 10C for the duration of the starting process. This might be 500 amps peak from a 50A-hr battery (for example).

I agree that the 'average C' for normal car batteries is obviously much less than 10C (based on the duty cycle), but I'm not sure if the 'average C' value is as relevent as 'C'. Average C can be made as low as you want by simply parking the vehicle for a couple of weeks. But it is still a ~10C event when starting.

(Duty Cyle is a dangerous concept unless the absolute periods are defined. '100 years ON followed by 1000 years OFF', is that considered to be 10% DC ?)

As a different example to buttress my point, diesel cars and trucks will use their battery to light up the glow plugs at perhaps 80 to 120 amps for perhaps 30 seconds (older cars longer, newer cars perhaps quicker), and possibly repeated several times on a very cold day, followed by a very difficult starting process (high compression engine, high current start). Diesel owners know that, on a very cold day, the battery might contain only enough 'juice' for a few start attempts. Three or five tries and then you're calling AA. The batteries will still last perhaps 4 or 5 years (YMMV) in this extreme application because they're designed for it.

But a 'deep cycle' battery might not last even one winter in a diesel truck. Not designed for it. Plates would probably buckle the first time you tried to crank over a big diesel engine at -40C.

This car battery business is exactly on-topic because it goes directly back to the OP's original question: "What is the limit...?" (referring to maximum current and lacking further details).

In the case of car batteries the perfectly reasonable limit might be as much as several hundred amps (provided you take into account all the obvious precautions and limitations).

And, the battery might last for many years in such an application (provided you take into account all the obvious precautions and limitations) even though the discharge rate peaks at about 10C.

I think that you're right about most batteries (0.1C is better), but car batteries are perfectly designed for very high current applications as per the OP's question (provided you take into account all the obvious precautions and limitations).

RE: Maximum output current for battery

"How do you get your car battery to last 8 years?"

20 years - 2 cars - 3 batteries (2 original, only ever bought one replacement battery). Current battery is original and seven years old. Seems fine.

And we do have winter in Canada.

RE: Maximum output current for battery

The 0.01C is the average C over the starting time only, not calendar time.  300A*5 seconds divided by 80Ah = 0.0052 of the total capacity.  Most cars will actually start in less than 5 seconds.  

TTFN



RE: Maximum output current for battery

I understood (and I thought that we had already agreed) that the units for 'C' is amps/amp-hours (or 1/time).

For your example: 300A divided by 80Ah = 3.75C

This definition of 'C' seems to agree with the information on the www regarding (for example) the recommended 0.1C charge cycle (0.1C is a ten hour charge cycle, plus a bit for losses, making about 14 hours).

Your definition makes 'C' to be unitless ratio (amp-hours divided by amp-hours = 0.0052) and I don't think that is correct.


RE: Maximum output current for battery

Lets see..
C or (Capacity)is in AH, the capacity of a battery, and is often stated based on some (arbitrary-ish) time.

e.g.  Battery X has a 7.2AH capacity based on a drain over the period of 20H.

This mean C is not based on an Hour but on a specified amount of time.

So a 1C drain would be 7.2AH/20H = 360mA drain.

A 4C drain would be 1.44A.

I'm seeing C is just amp hours (based on a specific amount of time).

See under characteristics the 'rates': http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_VRLA_LC-R127R2P.pdf#search=%22LC-R127R2P1%22


Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Maximum output current for battery

I want to know what brand those 8 year batteries are, and what type (deep cycle, high CCA, what).

I can make a mint selling those.

rmw

RE: Maximum output current for battery

The phrase 'deep cycle' may be misleading you, Don.

Those batteries are specifically intended to run your trolling motor for a day of fishing, and not intended to start your bass boat's huge outboard at the end of the day.

Regular lead-acid starting batteries _tolerate_ short term high rate discharge, and hours of 'float' service.

If you want batteries that actually thrive on high rate discharge, you want NiCds.  Even small ones can melt a wrench dropped on them, and survive it.

Mike Halloran
Pembroke Pines, FL, USA

RE: Maximum output current for battery

What EXACTLY do we mean when we write "0.1C" (for example) in reference to the current in or out of a rechargeable batteries???

If you Google: battery and 0.1C ...

Clicky ici: www.google.com/search?q=battery+0.1C

...you'll see that it is VERY common to specify a charge or discharge current (in amps) as a numerical ratio of the battery's nominal capacity (in amp-hours).

The units end up (most generally) as 1/Time, or depending on your point of view, maybe as amps (assuming that you know the battery amp-hour rating so that you can actually calculate the amps).

Thus a "0.1C" charge rate requires a nominal ten hours, plus about 4 more to make up for losses, and therefore typically about 14 hours.


This is why you SO VERY OFTEN see "14 hours" mentioned in the same paragraph as the term "0.1C".

Clicky ici: www.google.com/search?q=battery+0.1C+14+hours

(I got 18,000 hits...)


"0.1C" is certainly a bit of a units misnomer (not being amp-hours like the plain 'C'), but I didn't create it.

The misnomer:
   C = 7.2 AH
1.0C = 7.2 amps
1.0C <> C (yuck)

This certainly shows that whatever battery engineer invented the term "0.1C" didn't do very well in Grade 10 math...


A 1.0C discharge rate would discharge the battery in a nominal one hour (keyword: NOMINAL !!). Of course, as pointed out, it would actually be discharged significantly sooner because the battery only meets its rated Amp-hour capacity at some much lower discharge rate. Spec writers love the word 'nominal'.

So for the above 7.2AH example (current draws):
0.1C would be 0.72 amps (dead in just a bit less than the nominal 10 hours).
1.0C is 7.2 amps (dead in less than the nominal one hour)
4.0C is 28.8 amps (dead in MUCH less than the nominal 15 minutes)


Going back to the 80 amp-hour car battery example, you do NOT need to limit your current draw to "0.1C", or 8 amps, provided you've selected the right battery.

If you have the right battery, then feel free to draw 12C, or 1000 of them thar 'Cold Cranking' amperes, if you wish (provided you take into account all the obvious precautions and limitations).

The battery will still probably last for several years (even if you leave it outside in the extreme temperatures AND shake it around over potholes every single day).


Sorry to respond in such detail, but if we don't have a common understanding x.xC, and of the subtle difference between C and 1.0C in common (mis)usage, then we'll never understand each other.

Have a great day.

RE: Maximum output current for battery

"I want to know what brand those 8 year batteries are..."

1986 VW GTi - original battery probably Bosch.

About half-way along the 13 years, I bought a replacement from the dealer (got a good deal). Probably Bosch again.

1999 MB C230 - original (7 year old) battery still in the trunk (that's where it is, over the RR tire). I just checked the brand, it's a Varta.


Circa 1980, my 1972 Dodge Dart had an old Ni-Cad battery (surplus from a helicopter) for several years. Even at -35C it cranked up instantly (didn't run very well, but it certainly cranked well). Jump started half the neighbourhood on the coldest morning.

RE: Maximum output current for battery

I don't see many Varta batteries, but the few that I have had any experience with I liked a lot.  If I could find them I'd buy them every time.  Good pick.

rmw

RE: Maximum output current for battery

One included free with every 1999 MB C230...  winky smile

It's not only in the trunk, but it is also a bit bigger (longer) than the usual car batteries - perhaps just for weight over the RR tire.

RE: Maximum output current for battery

VE1BLL has explained it all very well. It is universal practice to rate the capacity of all batteries over the standard ten hour discharge period to reach the nominated end voltage.

That usually has absolutely nothing to do with the batteries intended application.

Something like a laptop battery, or the battery for a camera electronic flash unit will never be expected to survive anything like ten ours of constant normal use.  But a watch battery may be expected to last several years under continuous discharge. But both types of battery will be tested, and amp hour rated at the ten hour discharge rate (0.1C).

The answer is that the battery manufacturer has absolutely no idea what you are going to use his battery for, and a recognised standard for rating battery capacity is needed.

RE: Maximum output current for battery

Yuasa's SWL series are designed for short duration heavy discharge duties. They're common in smaller UPS applications where economics dictate that single or multiple VRLA strings are a better cost/benefit solution than wet cells. The data sheet gives some idea of their capability under short term heavy discharge.

http://www.yuasa-battery.co.uk/industrial/swl.html

----------------------------------
  Sometimes I only open my mouth to swap feet...

RE: Maximum output current for battery

http://en.wikipedia.org/wiki/Lead-acid_battery

Quote (From Wikipedia):

Lead acid batteries designed for starting service, such as those in used in most automobiles, are not designed for deep discharge. They have a large number of thin plates designed for maximum surface area, and therefore maximum current output, but which can easily be damaged by deep discharge. Deep cycling will result in capacity loss and ultimately in premature failure, as the electrodes disintegrate due to mechanical stresses that arise from cycling.

As discussed before, while auto batteries are designed for high peak currents, the durations are extremely short: 300 A over 5 seconds is only 1.5 kC, out of the total capacity of 80 Ah = 288 kC, so a single start is only 0.5% of total capacity.   

Contrast that against a 10 A discharge for 10 minutes from a 2.4 Ah battery = 69% of capacity.

TTFN



RE: Maximum output current for battery

Perhaps the whole thing with "C" rates was introduced since the capacity of a battery is a strong function of how it's used or how it can be used (LiIon versus NiCad for example), so to say simply that a battery has 10 A-h of capacity is meaningless without knowing the discharge characteristic.

http://www.batteryuniversity.com has a lot of information about batteries.


RE: Maximum output current for battery

True, but it is still a fairly good yardstick to compare the capacities of two batteries of similar chemistry but very different physical size.

Choosing the most appropriate battery technology to match the application in the first place, is an entirely different matter.

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