Rheostats in parallel. Load sharing??

[Ray Pooley]

Hi all.

Hyperthetical scenario.

Simple loop. 36V supply. 10 amps. So whatever resistor I place in the circuit to produce the 10 amps has to be capable of taking 360W. If I only have access to 200W capacity rheostats can I use two of them in parallell to share the load?

Thanks

 

[Lucien Nunes]

Yes. The power will be shared in proportion to the current, so if they are of equal resistance the power will be equal too. Two 7.2Ω in parallel or two 1.8Ω in series.

Curious... is this circuit doing any useful work? If all the 36V of the supply is across the resistance, what else is achieved? Or is this as a test load on the supply?

 

[Ray Pooley]

Yes. The power will be shared in proportion to the current, so if they are of equal resistance the power will be equal too. Two 7.2Ω in parallel or two 1.8Ω in series.

Curious... is this circuit doing any useful work? If all the 36V of the supply is across the resistance, what else is achieved? Or is this as a test load on the supply?

Thanks Lucien.

I figured as much. Thanks for the confirmation. No. It's not a working circuit. I have a neighbour who has an ebike and he's not happy with its performance. So he asked me to take a look at it. He has replaced the batteries. They are 6 x 6 volt SLAs in series. I checked all of the wiring and it's clean. It's only about a year old.

I fully charged the batteries and left the bike running on its stand for 8 hours (no load obviously) and just monitored the motor RPM and the battery life.

But I want to do a load simulation on the batteries. The motor rating is given as 360W to 500W. So I figure if I can use a resistor in a simple circuit to put a load on the battery pack and monitor the current with an in line meter I can see how well the battery pack can support the demand. I figured to it 360W and then 500W by varying the resistance.

500W rhestats cost a small fortune. But I can get a couple of 200W rheostats for about a tenner each.

I am just planning it at the moment to see if it's feasible or if it is a meaningul test.

Any thoughts would be appreciated.

Cheers

 

[Lucien Nunes]

Those are maximum power ratings. I doubt the batteries would support that load for long and I doubt it would be typical of bike usage. What capacity are they in Ah?

As an alternative to the rather intensive high-rate discharge test to deep discharge, SLA effective capacity can be tested at a lower rate of and the internal resistance can be measured. From these, the effective capacity to a known end-point terminal voltage can be calculated for any given load / discharge rate using the manufacturers' curves.
I use an electronic discharger that plots a discharge curve on the PC screen. They are meant for flight batteries and are now cheap and readily available. 20 years ago, before electric multirotor drones were the rage, I had to build my own discharger/analyser.

But, if the batteries have been replaced but the performance is not improved, is a test of the batteries actually helpful?

 

[Ray Pooley]

Those are maximum power ratings. I doubt the batteries would support that load for long and I doubt it would be typical of bike usage. What capacity are they in Ah?

As an alternative to the rather intensive high-rate discharge test to deep discharge, SLA effective capacity can be tested at a lower rate of and the internal resistance can be measured. From these, the effective capacity to a known end-point terminal voltage can be calculated for any given load / discharge rate using the manufacturers' curves.
I use an electronic discharger that plots a discharge curve on the PC screen. They are meant for flight batteries and are now cheap and readily available. 20 years ago, before electric multirotor drones were the rage, I had to build my own discharger/analyser.

But, if the batteries have been replaced but the performance is not improved, is a test of the batteries actually helpful?

"What capacity are they in Ah?"

That's one of the unknowns at the moment. I don't have the bike or the batteries. I know this could be a more sophisticated setup if I spend money on it. But it's not my bike so least spent the better. It may well be that the battery pack is actually under rated. Most modern ebikes are at least 40v and lithium. He bought it from China and it could just be crap. I don't know. What I do know is that the company who made it aren't trading any more. But I think it's an interesting project. I will work on the original battery pack first and then repeat the same exercise on the new one. If they produce teh same results then it's a case of c'est la vie.

 

[plugsandsparks]

You could also use 3 X 12V car lamps in series to give a good but not excessive load, loads of wattages to choose from.

 

[static zap]

You could also use 3 X 12V car lamps in series to give a good but not excessive load, loads of wattages to choose from.

Anything that glows , tends to self regulate , a near constant current effect (rather than current dropping as voltage drops )
..thinking ceramic car / lorry ? heaters..

 

[EalingBadger]

I use an electronic discharger that plots a discharge curve on the PC screen.

I could do with one of these.

What make and model are you using please?