Fixing Ego batteries in a low-voltage "defective" state
I returned home from a week of travel to find that (maybe somehow due to a power outage?) my Ego riding mower and/or its charger allowed the 6 battery packs inside to fall to an absurdly low voltage. Each pack measured around 22-24 volts, well outside of the allowable range of roughly 40-56 volts.
The visible effects of this were:
- The mower’s LED was blinking yellow (not orange or red)
- Each pack, when the power gauge button was pressed, blinked all LEDs in solid red
- “Resetting” the packs via two presses then a third long press didn’t fix them
- The batteries wouldn’t charge in the mower
- An Ego fast charger showed each battery as “defective” (with blinking red LEDs)
This is extremely annoying, given the cost of these batteries and Ego’s status & reputation as a Very Good, Upscale Brand for battery-powered stuff. (And it’s an indictment, IMO, of Ego’s [in]famous decision to leave basic battery management up to the tools & chargers. But anyway.)
Some Googling revealed that Ego would likely replace the affected batteries under warranty. This wasn’t good enough for me, because (1) I would like to use them today and (2) I bought a couple of them from eBay in open-box condition (because, again, these things are expensive).
One Redditor did have a plausible DIY suggestion: connect the dead battery briefly to a properly-charged one to bring its voltage up enough for the Ego fast charger to accept the battery and finish charging it.
Do not do this. This is not a tutorial.
You should not do this, unless you have experience with lithium-ion batteries and power electronics and you understand why this is a bad idea. Even then, this is still a bad idea, it's objectively dangerous, and you should not do it in your house. Also, wear leather gloves and eye protection.
And as a reminder, if you insist on doing this: double- and then triple-check that you're connecting the positive and negative wires to the correct battery terminals.
My first stab at this was trivially simple, using some pre-wired 5.5x2.1 and blade connectors I had already. This wire is approximately ∞ gauges too small and heated up very quickly (hence the recommendation for leather gloves):
That solution did work — after just a second or two the “dead” battery can go on the Ego charger just fine — but I decided it’d be smart less dumb to put a resistor in series so the system isn’t quite so much of a short circuit. Version 2 of the wiring includes a 10Ω 100W power resistor on a heatsink:
This solution works okay. It takes longer — around ten or twenty seconds — to bring the dead battery’s voltage up enough for the charger to accept it, which is probably a good thing. The wires don’t heat up noticeably, but the resistor certainly does.
If I were going to make a version 3, I’d probably want to buy something in the neighborhood of a 4Ω resistor and a bigger heatsink. And a heavier-duty DC connector, and something like 12-gauge wires. And properly sized blade connectors for making the battery connections.
And actually, first I’d actually do literally any math at all to size that resistor, heatsink, connector, and wire gauge, instead of hacking something together with literally the first parts I found in my basement.
But this did work.
Again, don’t do this.
(Before anyone asks, no, these batteries have never been used in my DIY Ego-powered 12 volt power supply, which anyway has a dedicated subsystem to prevent exactly this condition from developing.)