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Author Topic: Measuring amperage within a battery.  (Read 3481 times)

Captain Flack

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Measuring amperage within a battery.
« on: June 05, 2012, 01:44:50 pm »

How can I confirm the amperage within a battery to confirm the actual printed rating on the battery cover?  I have some 7.2v packs which show 7.2v+ when charged but I'm not sure if they are actually holding the 3800mah or equivalent?
Please try and keep any solutions very simple and in plain english :-) :-) :-) :-)please!!!
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rmaddock

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Re: Measuring amperage within a battery.
« Reply #1 on: June 05, 2012, 01:55:45 pm »

You could contrive some way to draw a known and constant current and then time how long it ran for 'till the battery was flat.
3800mah means that you can draw 3.8amps (3800 / 1000) for one hour. Or .38 amps for 10 hours. or 7.6 amps for 30 minutes etc etc.
You could put an ammeter inline with the battery and load (say a motor) and measure the current then do the sums.
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Tug Fanatic

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Re: Measuring amperage within a battery.
« Reply #2 on: June 05, 2012, 02:45:27 pm »

Be aware that capacity varies with load. This means that a battery discharged at 1amp will not last 10 times as long as one discharged at 10 amps. It will in fact last longer because the load is lighter. It might last say 12 times as long at 1amp. I have no proof of this but my impression has always been that the difference between low & high current is bigger the older the battery gets. The high draw load times go first.

The capacity that obviously matters most is the capacity with the load that you want to use.
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roycv

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Re: Measuring amperage within a battery.
« Reply #3 on: June 06, 2012, 08:57:43 am »

Hi, you mention 3800mAh cells, are these green and bought on the Internet?  If so then the numbers here are very suspect.  I bought some a year back and I think they are nearer to 2000mAh in practice.
Nimh and Nicads are given an amp hour capacity based on a current flow or discharge over a 5 hour period. (Called the 5 hour rate)  (Lead acid are graded against a 20 hour flow of current).

So your battery should deliver 0.76 amps or 760 m.amps  (milli amps) for 5 hours in theory.  You will not get 3.8 amps for 1 hour.
(3800 divided by 5 = 760)
When you take out a much higher current, the battery becomes less efficient and starts to get hot. And runs down quicker. This heat is produced from the current you would like to see going through your electric motor but is now being used to heat up the battery.

You can pay a lot or a little and guess what?  You get what you pay for!

The dearer batteries will deliver a higher current for short periods.  The cells cost more to make and are more effective at delivering the current.

When you charge a battery it is not 100% efficient.  You have to put in 40% more energy to restore the battery to full charge again.

If you discharge at the cell capacity in this case 3800 m.amps or 3.8 amps, then you will get maybe 50 % of the power out the rest going in heating the battery.

Hope this helps, regards Roy
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john s 2

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Re: Measuring amperage within a battery.
« Reply #4 on: June 06, 2012, 09:22:55 pm »

Roy has posted some very valid points about batterys. There is a lot of difference in quality and the ability to discharge at high rates . Do all manufactures stick to the five hour rule?  Could someone with more knowledge than me tell me are lipos more efficient in converting? I suspect  may be because the high amp discharge rate. But does this come at a capacity cost? Thanks John.
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Tug Fanatic

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Re: Measuring amperage within a battery.
« Reply #5 on: June 07, 2012, 07:58:30 am »

There are lies , damn lies & Lipos.

The whole business of cell capacity is fraught with danger & manufacturers claims that they cannot meet with the product. Modellers - mainly aircraft modellers - are very concerned about model weight and look for the smallest battery that will deliver the biggest current at the lowest weight and inevitably the lowest price. In market terms modellers don't buy many cells. Lots of other users want a cell that doesn't self discharge, holds a reasonable amunt of power & is cheap. This is the vast majority of the market.

There are cells advertised, both lipos & NiMh, that do exactly what they claim but there are a lot more that get nowhere near what they claim. Any cell that is charged or discharged more quickly than it is happy about will generate heat which will reduce the cell life considerably and in the ultimate destroy it immediately.

Lipos provide more capacity for a given battery weight that NiMh but there are no hard & fast rules about discharge efficiency rates. All cells have reduced capacity as discharge rates increase but the extent of this is very variable.

No not all manufacturers abide by the 5 hour rule.   
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Captain Flack

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Re: Measuring amperage within a battery.
« Reply #6 on: June 07, 2012, 12:26:03 pm »

Hi, you mention 3800mAh cells, are these green and bought on the Internet?


Yes they are, that's what prompted my post as I didn't think they were lasting anywhere near as long as they should.


You get what you pay for :-) :-) :-)
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essex2visuvesi

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Re: Measuring amperage within a battery.
« Reply #7 on: June 07, 2012, 12:34:08 pm »

When compared to a Genuine Tamiya 1700 battery pack these ones give almost double the run time so I would say the ratings are about right
http://www.ebay.co.uk/itm/7-2V-3800mAh-NiMH-Rechargable-Battery-x2-charger-tamiya-/250614050124?pt=UK_ToysGames_RadioControlled_JN&hash=item3a59c2ed4c
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roycv

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Re: Measuring amperage within a battery.
« Reply #8 on: June 07, 2012, 03:13:52 pm »

Having looked at the offer on ebay you can't complain, because to get 2 x  7.2 v batteries and a charger for less than £10 is good.  Even if the current discharge ability is suspect at the rate quoted

 Most of us are not looking for a 5 minute burst of power and then a re-charge, so I think I might just have some more!

OOPS  I retract that I have just seen the postage at £7.50.  I seem to remember paying £24 for 4 batteries with free postage.

Don't be fooled by the number of Hong Kong stamps on the packet, last time I looked the HK dollar was about 10p.  Sometimes the cost is low and the postage high to go through customs with a "low cost" item.

regards Roy
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roycv

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Re: Measuring amperage within a battery.
« Reply #9 on: June 10, 2012, 07:58:59 pm »

Testing the (green covered Internet) 3800 mAh battery

Hi, I have been doing some battery testing.  I have a Watts Up meter, obtained from our own sales and wants page.  I had checked it before and am confident in the numbers.

First, what is it?  The unit can make concurrent measurements of current and voltage and watts and also time.  This combined produces an absolute reading of amp hours or in other words battery capacity in one of the output figures.

I had done a more casual check of the Internet 3800 milli amp hour capacity cells previously mentioned and thought there had been a mistake, so I carefully charged the battery again and did a discharge test at around 1/2 an amp using a motor as the load.  In theory the battery should have been able to run for at least 7 hours on this discharge current.  Actual was 2 3/4. hours.
Remember that as the battery discharges the voltage of the battery reduces, it starts at about 7.9 volts and at below 6 volts it is about discharged.  That is 1 volt per cell.

The results were very disappointing at total measured capacity of just 1445 mAh capacity, as against the claimed 3800 mAhrs capacity. (less than 40 % of claimed capacity).   I took the load off and stopped the test when the motor was turning over at less than 2 watts.

Well I have 2 of these batteries and I had used them when new on a fastish model boat (a Bobby to those that know it) which planed nicely and took 4 amps under load with prop in the water etc.  About 30 watts. Just one battery fitted and the run time was about 25 minutes, i.e not full speed the whole time.  They both gave the same run time.

If you divide the measured 1.44 Amp hours by the current of 4 amps you come up with 21 minutes run time from the battery.  So even when new the same capacity as I have just measured.

I was recently given some "clapped" out batteries all 7.2 volts and I got one good set out of a pair of 3000 mAh batteries.  Not expecting a lot I used the same load as above and the new battery   came out at 2450 mAh.  Less than 10% loss of capacity over the years.  I do not know what a high current discharge would show.

These cells were expensive from a well known supplier (Component Shop).  They are probably 5 years + in age.
Like I said you get what you pay for.

Taking another view point you can probably buy 4 of the Internet batteries for the price of a one decent set of batteries.
You have to make your own decision there.

An interesting afternoon in the workshop.
regards to all, Roy
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essex2visuvesi

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Re: Measuring amperage within a battery.
« Reply #10 on: July 04, 2012, 07:40:45 am »

OK a little update to this.

Bought some more of the batteries I listed earlier in this thread as the old ones were starting to loose their "edge".  In their defence tho they are well used and have taken the punishment well
New batteries arrived and were charged up.  Run times are down quite considerably from the old ones even in their current "well loved" state, so one would have to summise that even tho they look to be exactly the same, the ones we bought earlier were in fact better quality, higher capacity cells.
I would say the recent batteries we bought would confirm Roys findings
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