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
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
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
htt
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Maximum output current for battery
Don
Kansas City
RE: Maximum output current for battery
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
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
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
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
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
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
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
TTFN
RE: Maximum output current for 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
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
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
Is this what you meant? That was what I was answering.
If I misinterpreted - sorry. Please clarify.
RE: Maximum output current for battery
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
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
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Maximum output current for battery
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
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
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
TTFN
RE: Maximum output current for battery
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
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': htt
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Maximum output current for battery
I can make a mint selling those.
rmw
RE: Maximum output current for battery
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
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
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
rmw
RE: Maximum output current for battery
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
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
http://www.yuasa-battery.co.uk/industrial/swl.html
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Sometimes I only open my mouth to swap feet...
RE: Maximum output current for battery
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
http://www.batteryuniversity.com has a lot of information about batteries.
RE: Maximum output current for battery
Choosing the most appropriate battery technology to match the application in the first place, is an entirely different matter.