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Mikey's HOW TO - iRobot dead cell battery fix...
I have three iRobots which help clean our house. I bought them about two years ago. As the one year warranty was coming up I called iRobot and asked them to help with various issues. They basically replaced all three of them. Now that I am at the one year point with the replacements I'm running out of options as things break. I can either fix it myself, or buy more shit. Obviously, I don't want to pay for anything. This morning I took apart the battery pack in the Dirt Dog vacuum cleaner. After about 30 minutes of cutting, sawing, and taping I had a battery pack that was performing like a new one. Now instead of getting 10 minutes of lousy vacuuming I get over a hour of powerful cleaning. This saved about $60 which is the cost of buying a new replacement pack from iRobot.
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Photos & Instructable on fixing the batteries...
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While this works for the short-term, in any situation where a battery pack is configured in a series, cell types should ALWAYS be the same. Cells have different discharge rates, resistances, and other tolerances. The voltages match, but current flows through the pack as a unit.
A cell with different characteristics can cause the other cells to discharge into it, or can discharge into the other cells. This creates a fire risk, and can cause overheating/leakage.
An entirely NEW battery pack constructed of new or used cells with similar characteristics would be a MUCH better long-term solution. These cells are sold for hobby purposes and are fairly cheap unless you buy the real high-end cells rated for super-high discharge rates etc.
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I've replaced a Roomba pack a few times. I bought one batch of just the cells off ebay and transplanted them into a battery case. The second battery I transplanted newer cells from the 1st dead battery to fix the dead cells. Seems to me like it was always cells 2 & 3 on one side next to the positive lead. The Roomba must have a voltage regulator here, and I think it (very) slowly cooks those two cells.
Rich is right tho, mismatched cells will discharge differently, and the AA cell can't match the sub-c cells for the amount of current it can handle before the voltage sags.
You might get away with it for a while, especially if you use a slow charger every now and then to "form" up the pack (bring all the cells up to peak. The Roomba isn't supper aggressive in discharge, so that helps.
D.
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I agree with David and Rich.
I bought a pre-wired replacement pack (I had to insert it into the old case) off ebay. I replaced the 3300 mAh NiMH pack with a 2400 mAh NiCd pack and it runs just as good as new (NiCd have slower self-discharge rates and are more robust than NiMH, just have to watch out for the memory effect).
I don't charge the battery in the robot any more due to the heating of the first couple of batteries. The roomba charging algorithm is terrible...it doesn't reduce to a low enough trickle, it just cooks the battery.
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As with Michael, David and Rich, I really must stress how BAD of an idea this is.
While this will 'work' I strongly recommend against it.
Replacement batteries are easily enough found and it is a lot cheaper to buy enough for two packs and then have one ready for when the one goes.
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