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### Author Topic: Simple Battery Question  (Read 397 times)

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#### Sylvestre

• Robot Overlord
• Posts: 221
##### Simple Battery Question
« on: December 30, 2013, 04:55:21 PM »
Hello,

A buddy of mine was trying to tell me that if I have two batteries in series to give me 24v at 55Ah that I should not draw more than 55 amps.

I said that you can draw more than 55 amps, its just that the run time will decrease.

Am I right or is he right?

#### Roman505

• Jr. Member
• Posts: 44
##### Re: Simple Battery Question
« Reply #1 on: December 30, 2013, 05:38:00 PM »
You are right about possible current draw, although if you are talking ordinary Lead-Acid batteries then actual run time should be a lot less, to preserve the battery. The "55 AH" rating given for such batteries is at 1/20th nominal discharge rate. They go flat a lot faster than seemingly indicated if used for higher currents and should not be driven below 50% charge anyway, for longevity, so beware of drawing higher currents, or even rated currents, from lead-acid for any length of time.

#### jwatte

• Supreme Robot
• Posts: 1,343
##### Re: Simple Battery Question
« Reply #2 on: December 30, 2013, 11:51:51 PM »
Batteries are generally rated with a "C" rating, which tells you how much you can draw as a multiple of the amp-hour rating. Thus, if you have a battery rated at 5 Ah, and it has a C rating of 20C, then you can draw 100 Amps from the battery (assuming the rating is proper and not "padded for marketing purposes.")

Most Lead/Acid, NiCad, and NiMH batteries don't really like being used with a current draw above 1C, although it varies a little bit based on specifics of the battery in question. See if you can get a specification from the manufacturer of your battery.

The other question is time of charging. Lead/Acid can be quick-charged to some extent. NiCd and NiHM don't like quick charging at all -- trickling at 1/10th C is often best for them, which means it takes forever to charge. Again, this varies somewhat with specific battery chemistry, so check the specs from the manufacturer.

LiPo and LiFePO4 batteries are generally made for very high current draw (C ratings from 3C to 100C) and reasonably quick charging (1C or 2C charge rate without damage.) However, Lithium-based batteries do not like being kept at their top charge rate -- if you want a "maintenance charge" or "float charge" then it should be at lower than max charge.

So, if you can tell us what type of battery you have, and ideally find the data from the manufacturer, we could answer your question much better.
For example, car batteries are sometimes rated with "CCA" (cold cranking amps) which is a measure of how many amps the battery can generate at the freezing point for some short amount of time. Often, that rating corresponds to a C rating of 10 or more -- but the actual capacity you get out of the battery at that draw is a small fraction of the rated capacity. You may run the battery down in a minute or two...

#### Sylvestre

• Robot Overlord
• Posts: 221
##### Re: Simple Battery Question
« Reply #3 on: December 31, 2013, 07:11:32 PM »
Hi,

First off, thanks for the responses.

I am using two NPC 1200 motors that draw 40a normal use each.

The batteries I'm using are Sonnenschein A512/55 Battery
Here is the datasheet
http://www.sonnenschein.org/PDF%20files/A500.pdf

#### Roman505

• Jr. Member
• Posts: 44
##### Re: Simple Battery Question
« Reply #4 on: December 31, 2013, 08:16:01 PM »
You can use them. Theoretical full discharge at 80A (two motors at nominal rating) would take about 30 minutes, not the 40 minutes suggested by the battery's 55 AH rating (at C20). Check Peukert's Law for more on this. It is on Wikipedia. I used a factor of 1.1 for the AGM battery.

Run time under those conditions would presumably be less if available voltage is to remain useable. Happily, those appear to be deep-discharge batteries so my "50% discharge rule" mentioned earlier will not apply directly.

Actual run time is closely related to the theory of linear string measurement. It depends on your usage pattern of course.