Run motor with insufficient amp batteries damaging?

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helpfulguy
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Run motor with insufficient amp batteries damaging?

Post by helpfulguy » Dec 04, 2017 10:11 pm

If you have 200A capable motor but only 100A burst/70A sustain batteries and 100A bms - will you risk damaging batteries and or bms, or will motor only get 100A?

Have 4x A123 cells 70A rated 200A burst, with 100A bms. Will connect to motorcycle starter motor which is somewhere between 70-200A depending on model. Want to avoid motor pulling 200A burning bms and/or drawing too much from batteries damaging them. Would rather just motor is weak with no damage.

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Drunkskunk
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Re: Run motor with insufficient amp batteries damaging?

Post by Drunkskunk » Dec 04, 2017 10:31 pm

If the BMS does it's job, it will shut down the battery at any draw over 100 amps. But it's designed as a safety device, not a means of current control, and i wouldn't rely on it. You should be relying on a controller, not the BMS to handle the current draw.
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Re: Run motor with insufficient amp batteries damaging?

Post by helpfulguy » Dec 05, 2017 11:16 am

Drunkskunk wrote:
Dec 04, 2017 10:31 pm
If the BMS does it's job, it will shut down the battery at any draw over 100 amps. But it's designed as a safety device, not a means of current control, and i wouldn't rely on it. You should be relying on a controller, not the BMS to handle the current draw.
Thanks drunk - what kind of current limiting device would be good here, while being reliable and keeping cost minimal?

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cycborg
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Re: Run motor with insufficient amp batteries damaging?

Post by cycborg » Dec 05, 2017 12:57 pm

helpfulguy wrote:
Dec 05, 2017 11:16 am
what kind of current limiting device would be good here, while being reliable and keeping cost minimal?
What is your application? Sounds like you're not using a controller, so is this just a replacement for the lead-acid battery in a normal gasoline motorcycle?

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Re: Run motor with insufficient amp batteries damaging?

Post by fechter » Dec 05, 2017 3:33 pm

With A123 cells, I wouldn't worry about it much as you won't be running the motor for long periods of time. If you draw too much current from any battery, it can overheat and possibly be damaged or catch fire.
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Re: Run motor with insufficient amp batteries damaging?

Post by helpfulguy » Dec 05, 2017 3:47 pm

cycborg wrote:
Dec 05, 2017 12:57 pm
helpfulguy wrote:
Dec 05, 2017 11:16 am
what kind of current limiting device would be good here, while being reliable and keeping cost minimal?
What is your application? Sounds like you're not using a controller, so is this just a replacement for the lead-acid battery in a normal gasoline motorcycle?
Correct! I want to use this as a learner project while also replacing lead for better alternative.

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Re: Run motor with insufficient amp batteries damaging?

Post by helpfulguy » Dec 05, 2017 3:49 pm

fechter wrote:
Dec 05, 2017 3:33 pm
With A123 cells, I wouldn't worry about it much as you won't be running the motor for long periods of time. If you draw too much current from any battery, it can overheat and possibly be damaged or catch fire.
Thanks fechter! Ok, so I hook up A123 cells with 100A bms board, and it should be good as replacement for short bursts?

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MadRhino
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Re: Run motor with insufficient amp batteries damaging?

Post by MadRhino » Dec 05, 2017 4:41 pm

helpfulguy wrote:
Dec 05, 2017 3:47 pm
cycborg wrote:
Dec 05, 2017 12:57 pm
helpfulguy wrote:
Dec 05, 2017 11:16 am
what kind of current limiting device would be good here, while being reliable and keeping cost minimal?
What is your application? Sounds like you're not using a controller, so is this just a replacement for the lead-acid battery in a normal gasoline motorcycle?
Correct! I want to use this as a learner project while also replacing lead for better alternative.
Then build a higher C-rate (or bigger) battery. For this purpose, the battery should reliably supply the full power demand of the starter.
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Re: Run motor with insufficient amp batteries damaging?

Post by dustNbone » Dec 05, 2017 11:20 pm

As long as it's just starting an engine, I doubt you're going to do any meaningful harm to A123 pouches. As long as the pack is properly built (solid connections between cells and output cables) it will probably be the death of the starter motor before the cells pack in. Heat is the potential enemy, and it will take some period of time before the cells reach a dangerous temperature.\

As long as 100A is enough to start the engine, the BMS should be fine. If it's not enough, then it just won't work. The BMS will shut down on overcurrent as soon as the starter engages, as it's highest amp load is when it's stationary. Once the engine is cranking over the current drops substantially.

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MadRhino
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Re: Run motor with insufficient amp batteries damaging?

Post by MadRhino » Dec 05, 2017 11:40 pm

Exactly. It will do with smaller starters, but the 200A will trip the BMS. Even the 100A starters can pull much more with a detuned engine, or sub zero temperatures.

It is easy to build a battery that is up to the task, the other reliable solution would be to gear down the starter.
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Re: Run motor with insufficient amp batteries damaging?

Post by helpfulguy » Dec 06, 2017 4:54 am

Gool Gool guys, interesting discussion! I made some more research:
  • Hard to find 4s bms with 200A+

    My 100A BMS will take 150A burst

    Some commercially starter batteries have these a123 26650 lifepo4 cylindrical cells without bms
Opens up discussion - maybe bms not needed? they wont unbalance I presume. Trickle charge no problem? Over charge?

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Re: Run motor with insufficient amp batteries damaging?

Post by miuan » Dec 06, 2017 5:06 am

Keep the BMS. The cells are fine with up to 200A burst. Make sure the rest of battery build is sturdy enough to sustain the peak current your starter will draw. Then build a simple external switching circuit, triggered by BMS output.

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fechter
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Re: Run motor with insufficient amp batteries damaging?

Post by fechter » Dec 06, 2017 8:51 am

If the BMS has issues with tripping, it might be possible to increase the tripping point or even disable the over current protection. Since the thing won't have to run very long, FET heating on the BMS shouldn't be an issue.

Also keep in mind a typical lead-acid car battery has somewhere around 50Ahr or more of capacity. If you want to run your audio system or lights with the engine off for any length of time, you need a decent capacity. A 10Ahr lithium battery might start the engine, but won't run the other loads for very long.
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