LiFePo4 "7th battery" on a lead bottom GEM

This was an idea that Dave @Inwo came up with at some point in the past and I was the first ̶i̶d̶i̶o̶t̶ scientific pioneer willing to test it…

… Adding an extra battery after the main contactor, so that only the high amperage terminals on the controller see the extra voltage. Translation, no voltage shunt needed for control power and that somehow changes how the controller sees things and the car performs and I’m gonna let Dave explain that part because he’s an electronics genius and I just fix stuff.

Yes, it’s really similar to Rodney @Old_Houseboater 7th battery 84V hack. Just different location, no shunt, and no disconnecting batteries to charge the main bank with a stock charger, since the contactor is open during charging and the two battery banks are isolated at that point. \

Took a while to get around to this because I decided to fix an input shaft bumper. This lead me to discovering I had a worn out input shaft with bad bearings and wounded motor. A new input shaft, new bearings, new oil seal, a new motor that didn’t freaking fit properly and a few emails to Rodney for some invaluable experience and advice and we’re back… (hide the sheep)

So here’s the idea, the circled 12v battery is an extra pack that we added after the contactor. It only provides extra power to the motor and only when the motor is in use.


So here’s how we did this (pay no attention to the man behind the curtain).

Tear the dash apart, find the output connector on the contactor.

Run a pair of new battery cables to wherever you plan to locate the new battery. I used a pair of 2 AWG and sleeved them in polyethylene wire loom braiding then installed connectors.

I have am '02 longback, the pack is going in the toolbox I have mounted in the bed.

(Getting dizzy yet from the pointless amateur Hitchcockan camera angles yet?)

Disconnect the output lead (that runs to B+) and connect it to what will become the battery + connection from the booster battery. Wrap it with proper insulating tape - this is going to see 300+ amps, you need to use good quality splicing tape, the 97-cent rolls of electrical tape just aren’t going to cut it here. This one got varnished cambric, 130C and then 33+.

Hook up the lead that will go back to battery minus to the contactor output.

Well, hook it up…

Battery hooked up and fast charger hooked up to the battery.

I wanted a bulkhead charger mount for the booster battery charger.

How cute…

Not 1.21 jiggawats, but hey, it works.

Remember, safety first. Always obey safety warning signs, as you might not get a second chance.

So with the wham-bam-bonus battery pack installed, and an R4F 7.5 hp motor, with 6 FLA batteries, 21" tires on stock 12" pizza cutter rims in a heavy azz 02 longback, I’m hitting 36-37 in the sprints with a good charge.

Super easy install, I spent more time figuring out how I wanted to install it and running around for the last minute extra part, than I did installing it.


Thanks for sharing. We only hear from the failed projects.:hugs:


Great minds think alike.

LOL. This one is actually working so far. Still determining the range and how often it needs to be charged vs the FLA bank.

What is the max voltage a T2 can handle? If we were to add 1 or 2 more cells to the pack, that would still put us below it, right? Probably means a new charger though?

Part of me is curious about seeing what an extra cell could do, other part got what I was after and doesn’t want to risk blowing up a controller.

Found some old posts I had made on the speed gains - all with 21.3" tires (12" stock rim)
stock (6 FLA + 5hp GE): 26 mph.
6 FLA + 5 hp GE + 4c pack + MM: 29-30 mph
6 FLA + 7.5 hp R4F + max speed limit raised on controller: 32 mph
6 FLA + 7.5 hp R4F + 4s pack + max speed limit raised on controller: 36 mph

Boxed up spare controller and sent it off to R4F this afternoon, told them I was running an extra battery for 84v, Curious to see what settings they put on it and what they do.


I think we might have even bought our stickers from the same company.

Search forum for T2 max volts. How would you charge 6s to 21 volts?

I thought at some point I had read that it was 96 or 99 - which is a big difference, basically one cell. My google-fu is failing me right now though, I can’t find ■■■■.

I would think that someone out there makes a 6s charger.

But looking at all the troubles Chris1 is having with his controller and 28s (98v) not sure I want to go there, as 6s booster would put me at 99.6v when everything is topped off.

Oh, sure if you want to buy a charger. I may have a custom one for about $100.
I believe it’s 95 or 96v max on T2.

Maybe a 5s. 6s would probably just blow something up.

Would love to see volt meters on both the lead and lithium while driving to see whats happening . Any idea when the pack gets low what the individual voltages are ?

Inwo convinces another ■■■■■ , I mean Scientific Pioneer , to try some crazy ass experiment .

I can see it now " Hey Barb , you’ll never guess what I got some SP to do today "

I’ll have to install the wiring and meters to get numbers while I’m driving, but Dave did send a mini plug in BMS -type thing that reads the lifepo4 bank and the individual cells.

After about 15 miles from last full charge, sporadic driving, some of it binary for fun, I hit 55% on the display. The FLAs were averaging 12.3v ea and the lifepo4 pack plug in thingimadodad showed 12.9v for the 4s pack and 3.22 give or take a little for each cell.

That’s pretty close . I would expect the Lithium cells would drain faster because of their lower internal resistance . This is a great upgrade , thanks for being the SP and sharing .

Got a code 77 error today.

“77 vehicle over speed error inspect motor controller”

Didn’t get it while driving the cart though.

Cart. Cart. Cart… no that’s not right… truck… no, not that either… * snap * I know… cart + truck = cauck. It’s a cauck. And a GEM of one too at that…

So, my cauck was parked for 8-10 hours resting, on charge (FLA pack) and lifepo pack was done charging a few hours in and the charger was unplugged. Unplugged the power to the Zivan, hopped in, turned on the “ignition”, tried to drive, boom - got the wrench & code. 2nd time this has happened, same exact circumstances.

Turned it off and back on, flipped the headlights on and just sat there for a few minutes. Not that my lights are going to draw anything, I converted everything to LED right after I bought my cauck.

Didn’t check the lifepo4 pack, but the FLA’s were hot… 13.7… real car alternator hot.

Few minutes later, everything was normal with my cauck so I gripped tightly, mashed down and was off to the races… And lo and behold, my cauck shot quick coming out of the hole with all that extra juice… at 97v.

I’ve only seen error 77 once, similar circumstances. Pulled my cauck off charge, tried to immediately do something with it. Got blocked.

Anyone know how the overspeed condition is determined? If pin 1 on the controller is way different from B+ is one thought I have, the other is that the RPM magnet on the R4F doesn’t like something that is going on. I never tested this extensively with the stock GE 5 hp motor, so I’m not sure.

Yes, easy enough to avoid, but would still like to know why my cauck isn’t working right…

77 is over voltage while running. Hard limit of about 96v. It protects batteries from over charging during regen.
T4 seems to have couple volts higher limit.

That makes sense. Thanks Dave

wow. I gotta let this one sink in.

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I think I’d just call it my trucklet.

I would never name my cauck, that’s way too assuming

So, some updates…

Max speed:
I realized I had a brake drum that was sometimes hanging up - with that sorta fixed, or at least not hanging up, I can hit 39 almost 40mph when everything is fully charged.

Based on Rodney’s posts, I reprogrammed #6 “Armature Acceleration Rate” from 60 to 35 and #11 “Turf Speed Limit” from 122 to 128. This gives me decent acceleration that doesn’t pull too hard on the batteries and a top speed in that mode of 25mph. Haven’t even started messing with fields and other settings yet. Now I drive around at 25mph in turf mode 90% of the time, with only the occasional flip to the high speed mode for going up freeway overpasses, tailgaiters and heavy loads. About 15-16 miles a day on it and it’s only down to about 80% power at the end of it all. Another 5 miles travel will take it down to 55%-60% and you really start to feel it getting sluggish.

Also decided to set the max overspeed at 35, which correlates to 6000 rpm on the motor with the tires I have on. I’m good with that. It was fun running at 39-40, but it was rough to push my cauck that hard.