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brucepick · 02-10-2012, 10:02 PM

Thank you to Electictracer and Daox.

My electrical skill is um, medium? I have more knowledge than an average person but absolutely don't feel up to designing and building a BMS. I've seen a couple writeups of what went into a BMS and I got lost almost immediately. Unfortunately I'm not ready to acquire half an Elec Engineering degree to get through the project.

I can solder and read simple circuit diagrams. I can calculate Ohms Law if I use a calculator (haha). I know what ohms, volts, amps and amp hours are which is very helpful on a project like this one.

If you can suggest a LiFePO4 pack of ~40-60AH, I'd be interested to see it. I do think the BatterySpace 40 AH pack is a pretty decent unit, more notes are below..

The BatterySpace 40 AH pack in post #1 is based on this pack, $245, sold by electricmotorsport.com. Anyway, BatterySpace also sells the $245 version which is without the PCM - protection control module. Electricmotorsport also sells some Thundersky stuff but not built up as a size pack I would need.

BatterySpace PCM: Protection Circuit Module (PCM) with Equilibrium Function for 4 cells (12.8V) LiFePO4 Battery Pack at 100A limited

Most importantly, the PCM does these jobs:

Keep 12.8V LiFePO4 Battery pack from overcharge ( 3.90 V/cell )
Balance each cell at Max. 3.60V/cell +/-0.03V ( required voltage tolerance within 0.2V for each cell)
Keep 12.8V LiFePO4 Battery pack from over-discharge (2.00V/cell)
Limit 12.8V LiFePO4 Battery pack's discharging current below 100 A.

Please see the link for all that it does.

Does it seem more sensible?

I feel I need an assembled pack with some kind of BMS, and of course a charger. The 40 AH pack would certainly start a car but the PCM limits current to 100A. Most days that would start it but on a real cold day with thick oil etc. etc. and everything working against you, the PCM limit of 100A might stop the show. BUT if it keeps the pack healthy without me needing an engineering degree, it might be a good way to go.

As noted by someone else in this thread, LiFePO4 should tolerate 80% DOD (20% remaining) and give ~2K cycles. Lead acid would only give a few hundred cycles at that rate. For this project, that's the main benefit of LiFePO4. With lead acid, I'd need maybe 150AH of battery to get a usable 30AH of charge, and ~200AH if I want >30AH. The weight and cost of that would be insane. The 40AH Li pack could give me 32AH without strain. And if I'm fortunate enough to be able to buy a second one a year from now, the pair would give a usable 60-64 AH, enough for many hour's drive.

02-10-2012, 10:02 PM	#6 (permalink)
brucepick OCD Master EcoModder Join Date: Dec 2007 Location: Eastern CT, USA Posts: 1,936 Outasight - '00 Honda Insight Team Honda Gen-1 Insights 90 day: 54.18 mpg (US) Thanks: 431 Thanked 396 Times in 264 Posts	Thank you to Electictracer and Daox. My electrical skill is um, medium? I have more knowledge than an average person but absolutely don't feel up to designing and building a BMS. I've seen a couple writeups of what went into a BMS and I got lost almost immediately. Unfortunately I'm not ready to acquire half an Elec Engineering degree to get through the project. I can solder and read simple circuit diagrams. I can calculate Ohms Law if I use a calculator (haha). I know what ohms, volts, amps and amp hours are which is very helpful on a project like this one. If you can suggest a LiFePO4 pack of ~40-60AH, I'd be interested to see it. I do think the BatterySpace 40 AH pack is a pretty decent unit, more notes are below.. The BatterySpace 40 AH pack in post #1 is based on this pack, $245, sold by electricmotorsport.com. Anyway, BatterySpace also sells the $245 version which is without the PCM - protection control module. Electricmotorsport also sells some Thundersky stuff but not built up as a size pack I would need. BatterySpace PCM: Protection Circuit Module (PCM) with Equilibrium Function for 4 cells (12.8V) LiFePO4 Battery Pack at 100A limited Most importantly, the PCM does these jobs: Keep 12.8V LiFePO4 Battery pack from overcharge ( 3.90 V/cell ) Balance each cell at Max. 3.60V/cell +/-0.03V ( required voltage tolerance within 0.2V for each cell) Keep 12.8V LiFePO4 Battery pack from over-discharge (2.00V/cell) Limit 12.8V LiFePO4 Battery pack's discharging current below 100 A. Please see the link for all that it does. Does it seem more sensible? I feel I need an assembled pack with some kind of BMS, and of course a charger. The 40 AH pack would certainly start a car but the PCM limits current to 100A. Most days that would start it but on a real cold day with thick oil etc. etc. and everything working against you, the PCM limit of 100A might stop the show. BUT if it keeps the pack healthy without me needing an engineering degree, it might be a good way to go. As noted by someone else in this thread, LiFePO4 should tolerate 80% DOD (20% remaining) and give ~2K cycles. Lead acid would only give a few hundred cycles at that rate. For this project, that's the main benefit of LiFePO4. With lead acid, I'd need maybe 150AH of battery to get a usable 30AH of charge, and ~200AH if I want >30AH. The weight and cost of that would be insane. The 40AH Li pack could give me 32AH without strain. And if I'm fortunate enough to be able to buy a second one a year from now, the pair would give a usable 60-64 AH, enough for many hour's drive. __________________ Coast long and prosper. Driving '00 Honda Insight, acquired Feb 2016. Last edited by brucepick; 02-10-2012 at 11:07 PM..