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LiFePO4 Lithium Iron Phosphate LeadAcidReplacement Batteries 
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Joined: Tue Jan 15, 2008 2:31 pm
Posts: 15371
Location: Home sweet home!
Some of you will know that I'm in the process of converting ALL of my cars over from Lead Acid traditional batteries to LiFePO4 based packs for a number of benefits:

1. Lighter
2. Much longer life/higher cycle capacity
3. No self discharge
4. More stable voltage (old power locks/windows work better with engine off)
5. Incredible current to capacity rating depending on cells
6. Better starting performance due to stable voltage
7. No acid vapour to rust old cars
8. Safe in a collision/puncture/overvoltage/overheat/etc - will not explode (Lead Acid is mostly safe, but during charging has H2 gas floating around which can/will give you an acid bath if not careful)

And likely other things!

The cell of choice for me is the Headway 38120HP battery with:

  • 8AH capacity
  • 300A CCA dead short discharge
  • 200A 30 second short term discharge
  • 160A max continuous discharge
  • 38x120mm cylinder form factor
  • 3.2V nominal results in 12.8V nominal with any 4S series configuration

Although not rated for it, they can also soak up alternator charge currents okay as I've been doing that to one pack for over a year and to another for several months without drama.

The systems I intend to use for these are as follows:

  1. 4 cells / 8AH for 1.6l or smaller engines
  2. 8 cells / 16AH for most of my cars (datalog from FreeEMS on 16v Volvo 240 showed excess drop while starting on 4 cells, but it did start, just a bit lazily - this is worst case with an old style inefficient starter)
  3. In-firewall isolator switch to totally turn the car off except for:
  4. different cells and larger capacity for Stagea to keep trip counter memory... grrrr
  5. 1A capable HeltecBMS brand 4S capacitor balance boards, small, cheap, efficient, low current drain (rated to 6A but cannot control a set of cells to 14.6V at 6A charge, first to fill goes over voltage badly
  6. URUAV MC-6S colour LCD cell voltage display, again low current draw
  7. 4PST switches for URUAV and balance board to totally disconnect the battery when not in use = 10+ years and you can show up, switch it on, spray some ether and start the car, awesome

Plus a couple of Fred specials to keep things simple...:

BMS for these things capable of running a starter motor are exceptionally expensive and not really needed except for under voltage cut off which I'm handling with the isolator anyway except Stagea which I'll just over-provision so it can't run flat from leak current quickly enough to fail.

But alternator current really needs handling for best longevity. Three parallel approaches:

  1. little 1A/6A capacitor balance board to maintain micro balance
  2. Precision medium current 3.65V clamp using precision V ref and grunty darlington or FET - stops cell over voltage during charging
  3. High-current zero drop FET diode for discharge to starter/vehicle with parallel resistor chosen to appropriately limit charging current (this will take some experimentation and I can do that now using a jump pack to start the car, a resistor and current shunt in series to measure charge current, and a near-flat LiFePO4 pack in the car to provide load to the alternator FET rating would need to be about 300-350A per 8A 300+CCA capacity

That last one was an idea I just had while thinking about the issue. So between those, and an isolator switch and the monitoring display to see if something has gone wrong, I should be good for reliable service.

For solar or boat use I'd do things a bit differently. Like the Stagea I'd over provision capacity even though I have started the 8 litre cummins with my 8AH LiFePO4 prismatic based jump pack (just!) and could almost certainly start it with 8 cells and start it pretty well with 12 cells. If for any reason extended cranking was needed to get the thing running, or multiple start/stop sequences during problem diagnostics, having more capacity would be a winner for sure. On the house side of the boat or for solar I'd use a traditional BMS that could balance, and isolate for under/over Voltage as necessary, or perhaps in the boat just an alarm on the low side and similar to the cars on the high side.

Given how much lighter these things are for the same capacity it's also possible to move them up higher in the boat so they're safe from water should she be sinking or leaking and splashing. Salt film discharge ruining expensive cells would be a very sad thing to have happen. Can also put them closer to where they're needed, and/or put some super capacitor banks at high draw locations to smooth the flow (winch, starter, pumps (maybe)).

I guess I should throw some media in here for you, one moment...










I should add that they ARE dangerous, but only in combination of:

1. How light they are - carry with one hand and other objects...
2. How exposed all the terminals are...
3. How much current they can dump out into your keys or other objects...

Yes, don't ask me how I know, but I did burn myself picking up the keys afterward :-D

Any comments welcome in this thread. No one uses forums anyway, so this is a diary/monologue ;-)

_________________ - where Open Source means Open Source, and Free means Freedom - the open source engine management system
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Sat Nov 07, 2020 7:09 am
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LQFP144 - On Top Of The Game
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Joined: Tue Jan 03, 2012 8:37 pm
Posts: 469
Location: Manhattan, KS. USA
Fred, thanks for keeping your progress documented here. My biggest concern is the safety aspect - as you've mentioned in detail. Is there a suitable and compact case that this can fit into which can sit comfortably in a hot engine bay on a summer day?

FreeEMS vehicle #23: 2003 Toyota "Toxic" Tacoma
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Sun Nov 08, 2020 9:05 pm
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Joined: Tue Jan 15, 2008 2:31 pm
Posts: 15371
Location: Home sweet home!
No worries, mate, need something like this for detailed write ups, they don't belong on short-format mediums like social media IMO, and email is too private/doesn't share well. so forums still reign supreme even if no one uses them any more except you and me :-D

About the safety side of it, I don't think I'd recommend using these in a bad heatsoaked engine bay scenario, however if you think about it, they're actually suitable for mounting up under your dashboard inside the car as they are a totally safe chemistry and don't vent dangerous acid and H2 gas :-D

The reality of most engine bays is just "warm" and that warmth increases the performance of the cells anyway, to a point. So that's not an aspect I'm too worried about. To that point, though, I'd rather have them out of a box exposed - for ventilation and inspection and for the looks - these do look even sweeter in person :-)

That only leaves the short circuit potential and if you look at the last photo, that's my first attempt to cover that aspect - a piece of HDPE/wood-fibre decking drilled to key into the plastic holders and fasteners so it can't slip off, a rubber sink sucker cup over the half-way / half-voltage terminals, and a piece of alloy bar bent to fit and clamped down firmly. What I used: ... h_p0267418

But, I'm not happy with it:

1. The decking is messy to work on and not actually very strong in small pieces, kind of crumbly
2. It's unnecessarily thick and heavy - like, quite heavy for a small bit of plastic
3. The rubber degraded and cracked badly after only a few months of oxygen supply and sustained deformation
4. Too tall for what it does.

I feel like some sort of medium-temp-resistant 3D printed tray for the bottom and part tray / strapping saddle for the top would be a winner. I would be tempted to use wood, but wood doesn't like getting wet repeatedly and could also split.

In the Suzuki it already had a plastic tray, so I just sat it in the middle of that and put a big HD cable tie over the top through the connections on a diagonal and that's been great and is amply strong. That's a 4 cell pack, though, so only 1.4kg and restrained by the wire connections somewhat at the top, too.

Do you own a Prusa Mark 3 or later? :-D How're your CAD skills?

_________________ - where Open Source means Open Source, and Free means Freedom - the open source engine management system
FreeEMS dev diary and its comments thread and my turbo truck!
n00bs, do NOT PM or email tech questions! Use the forum!
The ever growing list of FreeEMS success stories!

Sun Nov 15, 2020 9:55 am
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