Maximum output current for battery 4

[eromlignod]

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

 

[VEBill]

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).

 

[VEBill]

"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.

 

[IRstuff]

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

 

[VEBill]

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

http://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.

 

[itsmoked]

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...onic_VRLA_LC-R127R2P.pdf#search="LC-R127R2P1"

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rmw]

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

 

[MikeHalloran]

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

 

[VEBill]

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:

http://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:

http://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.

 

[VEBill]

"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.

 

[rmw]

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

 

[VEBill]

One included free with every 1999 MB C230... ;-)

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.

 

[Warpspeed]

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.

 

[ScottyUK]

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...

 

[IRstuff]

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

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

 

[jimkirk]

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.

 

[Warpspeed]

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.