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Old 04-16-2014, 05:07 PM   #31 (permalink)
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Resolution has to be lost. Discounting ADAD conversion errors, your input from the one board is in 1024 distinct steps, but the output from that board is 256 distinct steps. and it isn't 4.5v/256 it is the cells voltage above relative ground that is sent.

I assure you that you wont regret sorting out serial communication They have a two port example for software serial built right into the arduino ide.

Instead of pwming, you just write the converted adc values over serial, with no loss, plus you can control other serial devices.

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Old 04-16-2014, 07:04 PM   #32 (permalink)
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I assure you that you wont regret sorting out serial communication
I agree. However, is it possible is isolate serial transmission through optos (or equivalent)? Arduino boards will have to be isolated to avoid voltage potential differences (at different parts of the pack's series string) from wrecking stuff. Since serial comm goes both ways down a wire, a simple opto is no good there. My PWM comm should work through an opto because one wire goes from Master to Slave and the other from Slave to Master, never changing directions.

There must be another type of IC that does two way isolation.
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Old 04-16-2014, 07:34 PM   #33 (permalink)
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yah opto's are doable, the slow ones are like $0.20 a piece, if you can live with a slower baud rate they are fine.

http://www.mouser.com/ds/2/239/775704_1-268944.pdf

Faster ones cost a bit more and have larger power requirements.

FYI, in the "one cpu per cell" model I am working on only using them on the ends of the battery pack, and node to node communication uses a level shifter. But per cell cost is important, and it should be fast and still operate within the expected voltage range.
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Old 04-18-2014, 12:09 PM   #34 (permalink)
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(downloading coocox ide + prereqs)

Yah, I'm thinking that soldering and wiring are going to be challenging if you don't understand this stuff. And being all experimental when you are trying to protect your batteries could screw up something, do more harm than good.

You know how to measure voltage? That is the only skill needed for bottom balancing.
Yeah, I know how to measure voltage. I know how to use a multimeter to test voltages and resistance and things like that.

So you guys think if I don't have a BMS I should bottom balance once in a while and that'll be good? By once in a while I mean once every few months.
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Old 04-18-2014, 12:15 PM   #35 (permalink)
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If you want a fool proof system you want a BMS. If you want to tinker and tweak, then you can get away without one.
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Old 04-18-2014, 12:29 PM   #36 (permalink)
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So you guys think if I don't have a BMS I should bottom balance once in a while and that'll be good? By once in a while I mean once every few months.
Like Daox said.
The biggest issue is knowing when to stop the charger. If you bottom balance, the weakest cell will hit max voltage at the top of the charge before the others. If you continue to charge, it will go into the danger zone while the others catch up. So at minimum you should have a device that measures the voltage of your weakest cell and stops the charger when it reaches max voltage.
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Old 04-18-2014, 01:04 PM   #37 (permalink)
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Yah we haven't discussed charging, or even how a bms interfaces with a charger (or a controller)

Here is a good video about charging manually as well. Eventually you want to get everything down to the cell level and automatic, but you can at least get rolling cost effectively, and 18 cells is more manageable manually than others
EVTV Motor Verks | Electric Car Conversion Videos

The leaf cells are a bit trickier than the lifepo4 cells fyi, they might not be as resistant to overcharging, and the special voltages of the leaf cell are limiting your cost-effective bms and charger options, so you might want to consider lifepo4 too.

Last edited by P-hack; 04-18-2014 at 01:13 PM..
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Old 04-18-2014, 02:26 PM   #38 (permalink)
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Yah we haven't discussed charging, or even how a bms interfaces with a charger (or a controller)

Here is a good video about charging manually as well. Eventually you want to get everything down to the cell level and automatic, but you can at least get rolling cost effectively, and 18 cells is more manageable manually than others
EVTV Motor Verks | Electric Car Conversion Videos

The leaf cells are a bit trickier than the lifepo4 cells fyi, they might not be as resistant to overcharging, and the special voltages of the leaf cell are limiting your cost-effective bms and charger options, so you might want to consider lifepo4 too.
I was looking at the (recommended to me for the leaf batteries) the Elcon 2500 charger, which can be programmed by the supplier. So I'm getting recommended charging programs from the battery supplier.

I'm still just concerned with the BMS. I'm uneasy about the DIY route because of time and my experience with frying electronics. But I can't find any good affordable BMS systems otherwise.
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Old 04-18-2014, 04:03 PM   #39 (permalink)
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The elcon looks like a cost effective unit for known battery types. But the default cell count and voltages do not look suitable for your special chemistry batteries (LiMn2O4), so you will need to have it reprogrammed, or order direct with a suitable charge profile. I haven't seen evidence of diy programming on those units.

Edit, maybe I stand corrected: $355 and LiMn2O4 is an option at 72v
http://www.electriccarpartscompany.c...lay_p_349.html

Last edited by P-hack; 04-18-2014 at 04:36 PM..
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Old 04-18-2014, 04:55 PM   #40 (permalink)
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The elcon looks like a cost effective unit for known battery types. But the default cell count and voltages do not look suitable for your special chemistry batteries (LiMn2O4), so you will need to have it reprogrammed, or order direct with a suitable charge profile. I haven't seen evidence of diy programming on those units.

Edit, maybe I stand corrected: $355 and LiMn2O4 is an option at 72v
1500W 1.5kW 12V, 24V, 36V, 48V, 60V, 72V Output 1500W 1.5kW LiFePO4 Li-Po Lithium Battery Charger 1500W 1.5kW Lead-Acid Battery Charger 1500
I'm wondering how I didn't see this charger before. I go to that site frequently for part references.

The BMS is still annoying me, I guess because I'm still not 100% sure what to look for in a BMS.

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