Absolute Cheapest Load To Test A Rebuilt Battery

Need to draw 18 Amps from a 36 v battery.

For 5 minutes.

2 lugs in a brine solution?

Drive rebar into the ground?

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Bret Cahill
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Well, that's 2 Ohms, 648 Watt.

Brine works, although you must measure the Amps and regulate the electrodes manually. Make sure not to short circuit the current. One bucket of water, 10 liters, and 1/4 Kg of salt.

40 cm of aluminium rod electrodes. Starter cable of the Olde Times, not the Chinese ones.

Done this a 1000 times.


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Not sure if the sprinkler system is de facto flood irrigation or not. Probably get pretty good current with all the Ca and Na in lower Colorado River water.

Meter only goes to 10 amps so play with that then extrapolate off when the 15 amp fuse blows.

Is this an inverse square relation with distance?

If concentration is linear if may be easier to 3X the salt of 6.7 amps.

Two 15 amp fuses in parallel.

I might use something taller to accommodate the rods.

Get $1.25 of Na salt as I'm not going to waste $3 of lite salt on this project.

The two Al angles happened to be 1 m away cluttering the desk, one 35 cm and one 40. I may cut the test after 2 min if they corrode too much. I'm a great believer in useless clutter.

I'll just use the 4 X 16 AWG I use for the spot welding rig. In 2 years I'll replace the cells again so I'm not going to ruin my cables.


The 40 new cells were left over from a 7p X 10s cut to make a 3p range extender and were sitting for 7 months. They are now are all at 32 v. I'll charge up half way with the BMS attached then discharge test a minute w/o BMS. Then 5 minutes with BMS.

The bike stays out doors until after several charge cycles.

I stopped by the Glamis Store asking if I could use their outlet to recharge.

"We are not responsible if your bike burns up."

They didn't care if if the ebike burned their wooden store down, just getting sued. It took awhile to realize they got sued everytime anyone flips a dune buggy.

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Bret Cahill

Three thrift store kettles in parallel?


Phil Hobbs

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Phil Hobbs

Might as well heat up some bath water so the energy isn't completely wasted.

Reply to
Bret Cahill

The angle had been mangled in previous projects so the electrodes could not be positioned close together enough to get below 20 ohms. I had to spend $5 for lump charcoal. (Always try these things outdoors next to the grill with some steaks in case of a fire.) Adding pulverized charcoal to the brine, or rather, adding the brine to the charcoal, got the "resistance" down below the equivalent of 2 ohms. It's not really a pure resister so the ohm meter was worthless. I measured current from a 3.6 volt battery which was ~ 1 amp. This isn't an ohmic load so when I hooked up the 36v battery it drew 18 amps for the 1 or 2 seconds I was willing to keep the test going.

Eventually I raised the electrodes out of the slurry until it drew less current.

I blew a 15 amp fuse and used 1 cm wide al foil instead. Steam soon boiled out the top of the sch 80 PVC pipe container but I held in there for a full minute. No hot spots anywhere on the battery.

I had charged up 0.6 v earlier -- half an hour at 1.18 amps -- and the voltage dropped 0.4 v in about the minute and a half I was actually zapping the thing.

This comes out to be an average of 18 amps.

I'm not going to bother with the 5 minute test with BMS. Hardly any hills in the valley take much more than a minute anyway.

The original BMZ looked like a prototype, like no one had done a thorough analysis of the possible wear from possible vibration that could short the battery. So BMZ adopted a throw eberthang ya got at them approach. The battery was held together by dozens of cool tapes and sealants -- a Maginot Line.

Now I'm starting to wonder if I took enough care. My reasoning was the down tube should be the battery holder. Stuff that battery tightly enough in the down tube and there is no intra battery rubbing. Sure would hate for it to short out a year from now.

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Bret Cahill

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