Burn Testing Scooter Batteries. A couple of questions

[And]

Hi

I have been burn testing my scooter batteries today with a new burn tester that Rick Huggins made for me (thanks Rick). I believe that the resistance is supposed to be 1.5 ohms, although on my multimeter it seems to indicate 2.0, ie the reading is 02.0 when set on the 200 ohms range. Is that 2.0 or am I taking the reading wrong?

Not knowing much about electrical measurements and stuff, is resistance linear? Can I take my burn time and extrapolate a figure based on a 1.5 ohms resistor?

The burn time was about 35 minutes, taking the battery pack to 19 volts or so (bloody telephone call just at the wrong moment) whereas I am supposed to get between 40 and 50 minutes, with the correct resistors, before the pack drops to 20 volts. I know that the batteries easily last 45 minutes to an hour when actually scootering, but I think I might need to get a new battery pack and test them with this resistor to get a feel for what a good test is.

Actually, I think I have just answered my own query but what do you guys think?

Andy

[Janos]

Originally Posted by And

Hi

I have been burn testing my scooter batteries today with a new burn tester that Rick Huggins made for me (thanks Rick). I believe that the resistance is supposed to be 1.5 ohms, although on my multimeter it seems to indicate 2.0, ie the reading is 02.0 when set on the 200 ohms range. Is that 2.0 or am I taking the reading wrong?

Not knowing much about electrical measurements and stuff, is resistance linear? Can I take my burn time and extrapolate a figure based on a 1.5 ohms resistor?

The burn time was about 35 minutes, taking the battery pack to 19 volts or so (bloody telephone call just at the wrong moment) whereas I am supposed to get between 40 and 50 minutes, with the correct resistors, before the pack drops to 20 volts. I know that the batteries easily last 45 minutes to an hour when actually scootering, but I think I might need to get a new battery pack and test them with this resistor to get a feel for what a good test is.

Actually, I think I have just answered my own query but what do you guys think?

2.0 does mean 2.0 ohms. So yes you're doing it right.

Voltage (V) = Current (I) x Resistance (R)
so 19 Volts = 2.0 I
so you're pulling 19/2 = 9.5 Amps with your resistor.

If you had the 1.5 Ohm resistor in then you'd pull

19 Volts = 1.5 x I, so I = 12.7 Amps

12.7 is one third bigger than 9.5 Amps, so I would expect your battery to burn for 1.5 times longer with the 1.5 Ohm resistor in. So 47 minutes instead of your measured 35 minutes. Which is about right.

So Physics says that your battery is fine, but engineering says just get another one and test that as it will give you a better feel ("empirical evidence" if you're being posh) and it would be handy to have a spare in your Shed anyway.

Janos

[And]

Hi

Thanks Janos. Perfik

Andy

[RichW]

Purely empirically, if your load resistor is 2 Ohms, it will draw less current than one of 1.5 Ohms.

Consequently your burn test times will be OVERestimated.

Also, be aware that a resistor will change it;s value when it gets hot. Did you measure them hot, or cold?

Rich

[Janos]

Originally Posted by RichW

Purely empirically, if your load resistor is 2 Ohms, it will draw less current than one of 1.5 Ohms.

Consequently your burn test times will be OVERestimated.

Bollocks, bollocks, bollocks. Rich is right and I am wrong. I even said that the 2 Ohm resistor draws more current.

That will teach me not to check my working.

Janos M.Phys

[And]

Hi Rich

Measured them hot and cold, and there is no difference.

But if I know that in the 'real' world, I can scooter for 45 mins with stage bottles etc. then why is my burn test only 35 mins and overstated ?

Andy

[RichW]

Andy,

Maybe the burn tester is designed to be conservative. Or reflect someone hauling multiple stages, scooters etc.

Rich (PhD, Phys )

[And]

Hi

The burn tester is made up similar to the ones in the Gavin manual, except these resistors are 3.1 300W instead of 3.0 300W.

I am sure that it is just a case of measuring new batteries and using that as a benchmark, when burn testing other battery packs, but I was just curious why my burn test would be so far off.

I'll do it again. Its Friday

Andy

[SimonK]

Testing a battery pack like this is good for checking how the battery capacity degrades over time or for compairing one pack against another. It wont tell you much about how it behaves in the real world, too many variables in the real world load. Its a bit like looking at official fuel consumption figures for a car.

Simon

[RichW]

Originally Posted by And

Hi

The burn tester is made up similar to the ones in the Gavin manual, except these resistors are 3.1 300W instead of 3.0 300W.

I am sure that it is just a case of measuring new batteries and using that as a benchmark, when burn testing other battery packs, but I was just curious why my burn test would be so far off.

I'll do it again. Its Friday

Andy

Is this the first time you've burn tested them?

Are they old batteries? How old, and how many charge/discharge cycles have they had?

Rich