Which diodes can support 60V 30A? (For Longer Range)

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Quentin_Rufin   10 µW

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Which diodes can support 60V 30A? (For Longer Range)

Post by Quentin_Rufin » Jul 11 2018 11:41am

Hi!

I am trying to extend my ebike range by putting a diode to prevent early cut off voltage from the BMS.

I have a BBSHD with a 14s10p made of LG MJ1 cells. The motor generates a current of 2 to 4 volts (at 30A) which trick the BMS sooner than expected.

Therefore, I was thinking to add a removable diode between the controller and the battery in order to use the battery down to 2.6v per cell or 37v (Lowest BBSHD controller setting) when battery voltage is around 3 volt per cell.

I found this diode on st.com STPS61L60C. However, I feel that I am taking the wrong path when I see how thin the wires are on the diodes.

Do you think I can use 4 10A shokty diodes in parallele?

Any thoughts?

lionman   100 W

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Re: Which diodes can support 60V 30A? (For Longer Range)

Post by lionman » Jul 11 2018 12:47pm

Maybe a diode module would be better suited than a through hole if you are planing on using it inline.

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fechter   100 GW

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Re: Which diodes can support 60V 30A? (For Longer Range)

Post by fechter » Jul 11 2018 2:34pm

I don't understand what you are trying to do with the diode. Placing a diode between the battery and the motor won't change much.
"One test is worth a thousand opinions"

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amberwolf   100 GW

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Re: Which diodes can support 60V 30A? (For Longer Range)

Post by amberwolf » Jul 12 2018 1:14am

All the diode will do is lower the voltage seen at the controller (more voltage drop the higher the current).

If your controller has it's own LVC, that could cause it to shutdown earlier than it normally would.

The diode is also going to get mightily hot; possibly requiring a fan and heatsink. (the fan will have to be powered by something, so you'll use up power out of your battery pack that would've given you more range).


That said, running your cells that far down is a bad idea--they'll get much more unbalanced than otherwise (recharging will take much longer, potentially days longer, as you wait for them to rebalance), and will age the cells faster, so your battery will need replacement sooner. (possibly a lot sooner)

If you want more range without damaging what you already have, get anohter battery to parallel with the one you have--not only will you have more range from the extra capacity, but also becuase each battery will be hammered less than if it was providing the whole load, so there will be less sag and you'll get more of the total power each has available before it shuts off from sag,

Quentin_Rufin   10 µW

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Re: Which diodes can support 60V 30A? (For Longer Range)

Post by Quentin_Rufin » Jul 12 2018 11:21am

fechter wrote:
Jul 11 2018 2:34pm
I don't understand what you are trying to do with the diode. Placing a diode between the battery and the motor won't change much.
When you use a DC motor it generates a current on the opposite direction. This is called back EMF.
This current has a voltage of between 2 and 4 volts with the BBSHD. Placing the diode when the 14s battery voltage is around 41v will prevent early LVC from the BMS. However, it is counter productive above 41volt.

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Re: Which diodes can support 60V 30A? (For Longer Range)

Post by Quentin_Rufin » Jul 12 2018 11:21am

lionman wrote:
Jul 11 2018 12:47pm
Maybe a diode module would be better suited than a through hole if you are planing on using it inline.
Do you have a link?

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amberwolf   100 GW

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Re: Which diodes can support 60V 30A? (For Longer Range)

Post by amberwolf » Jul 12 2018 12:30pm

Quentin_Rufin wrote:
Jul 12 2018 11:21am
When you use a DC motor it generates a current on the opposite direction. This is called back EMF.
This happens in any motor, but it doesn't have anything to do with the battery voltage--it's all on the motor side of things.

A diode between battery and controller will not do anything other than lower the battery voltage seen by the controller by the Vf of the diode (reducing the power the controller can receive), and waste some power (range) as heat in the diode.

It won't change the BMS behavior. The BMS will still cut off at the same voltage it's designed to, as that's based on the individual cell voltages.


The diode *will* prevent any of a regen-capable system's back-EMF from being able to recharge the battery, preventing you from recovering any energy and causing the system to have less range than it could otherwise have.

Your system wouldn't have any regen energy recovery anyway, since it's got freewheels between ground and motor, but if you did have such a system, like a DD hubmotor, you'd get *less* range with a diode there, not more.

nfmisso   10 mW

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Re: Which diodes can support 60V 30A? (For Longer Range)

Post by nfmisso » Jul 12 2018 5:59pm

Here is a diode that will take the load (actually two in one package).

If you get two batteries in parallel, each running through one side of this diode package, you'll be able to prevent one battery from charging the other, allowing safe use of two batteries of widely differing capacities.

As amberwolf noted, what you propose will not do what you want it to.

nfmisso   10 mW

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Re: Which diodes can support 60V 30A? (For Longer Range)

Post by nfmisso » Jul 12 2018 5:59pm

https://www.amazon.com/dp/B011OM448Q

Sorry here is the link to the diode package.

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Re: Which diodes can support 60V 30A? (For Longer Range)

Post by dustNbone » Jul 12 2018 6:11pm

I'm pretty sure the 2-4V drop in battery voltage that you're seeing at 30A draw is just plain old sag, caused by internal resistance of the battery. Ohms law and such.

The back EMF doesn't make it out of the motor in these circumstances, it's just pushing back against the current entering the motor, the net flow is still in the direction of Battery -> Motor.

Nothing for a diode to do except get hot and waste energy.

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