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Greg Fordyce · 12-06-2009, 10:29 AM

I have been involved in a open source lithium bms project for over a year now. Probably the hardest and most crucial thing is how you use the system to prevent cells being damaged by over discharging them. The only way to do this with most controllers is to reduce the throttle input. It's tricky to do though as you don't want to abruptly shut it off as that could be dangerous but you do need to keep reducing it at a level that prevents any cell going below it's allowed minimum voltage.

So this project presents an opportunity to solve this problem. It would require a input to the ATmega168, but I understand that a few are available. The idea is this, we have a input for low pack voltage. This is supplied by the bms, but it could even be useful for lead acid conversions, all you need is a signal when pack voltage drops below a certain level. When the controller receives this signal it calculates battery amps, which I believe is already done in software, and sets a new value for maximum allowed battery amps to be say 10 amps lower than the present value. So if you are drawing 200 amps from your battery pack when the bms signals a low cell warning, the controller will reduce this to no more than 190 amps. If this still doesn't put off the alarm then it drops to 180 amps, etc all the way to 0 amps. Of course backing off the throttle will also put the alarm off, but the controller will remember its max allowed battery amps and will never allow battery amps to exceed the maximum until it is switched off. Only then will it reset back to it's default value.

The effect when driving is that the car will start to gradually slow down when the battery pack is nearly depleted. By taking it easy you can maybe gently nurse the car to your destination, or at least a place to plug it in. But it will never allow you to over discharge a battery, which shortens the life of lead acid and murders lithium cells.

BTW, link to the bms project I mentioned can be found here;
Battery Vehicle Society :: View Forum - All things battery related
Look for the top 3 sticky topics, BMS General, BMS Hardware and BMS Software.

Also, where can I find the latest controller software?

Greg

12-06-2009, 10:29 AM	#2693 (permalink)
Greg Fordyce EcoModding Lurker Join Date: Nov 2009 Location: Scottish Borders, Scotland Posts: 92 Thanks: 7 Thanked 33 Times in 16 Posts	Battery management and the Open Revolt controller I have been involved in a open source lithium bms project for over a year now. Probably the hardest and most crucial thing is how you use the system to prevent cells being damaged by over discharging them. The only way to do this with most controllers is to reduce the throttle input. It's tricky to do though as you don't want to abruptly shut it off as that could be dangerous but you do need to keep reducing it at a level that prevents any cell going below it's allowed minimum voltage. So this project presents an opportunity to solve this problem. It would require a input to the ATmega168, but I understand that a few are available. The idea is this, we have a input for low pack voltage. This is supplied by the bms, but it could even be useful for lead acid conversions, all you need is a signal when pack voltage drops below a certain level. When the controller receives this signal it calculates battery amps, which I believe is already done in software, and sets a new value for maximum allowed battery amps to be say 10 amps lower than the present value. So if you are drawing 200 amps from your battery pack when the bms signals a low cell warning, the controller will reduce this to no more than 190 amps. If this still doesn't put off the alarm then it drops to 180 amps, etc all the way to 0 amps. Of course backing off the throttle will also put the alarm off, but the controller will remember its max allowed battery amps and will never allow battery amps to exceed the maximum until it is switched off. Only then will it reset back to it's default value. The effect when driving is that the car will start to gradually slow down when the battery pack is nearly depleted. By taking it easy you can maybe gently nurse the car to your destination, or at least a place to plug it in. But it will never allow you to over discharge a battery, which shortens the life of lead acid and murders lithium cells. BTW, link to the bms project I mentioned can be found here; Battery Vehicle Society :: View Forum - All things battery related Look for the top 3 sticky topics, BMS General, BMS Hardware and BMS Software. Also, where can I find the latest controller software? Greg