I have a huge arsenal of Roombas. I think the count is up to 3 400 series Roombas, 1 500 series Roomba, 1 Dirt Dog and 2 Scoobas. I love having lots of these robots to clean and vacuum my house and houses that I sell too. As a result, I’ve gone through a LOT of batteries. What I’ve started to do is write dates on the battery of when I purchase them. If there is anything else special or substantial(such as used or li-ion) about the battery, I’ll write that on it too.
Looking to gain more run time and a longer cell life, I’ve tried out several Lithium Ion battery packs. If you’ve shopped for these, you’ll know that they are substantially more expensive than the plain NiMH batteries. I’m here to tell you that they are NOT worth the extra money. If I could buy a Lithium Ion pack for a 400 series Discovery for the same price as a NiMH, I might do that but otherwise I would advise against it. There are specifics for each robot as to why they are not well suited for use with a Lithium Ion pack. I will detail that below but the long and short of it is that the Roomba charging circuits are NOT designed to optimally charge a Lithium Ion pack. Most of the time the cells are being charged to 4.3v or maybe even higher. If you drop the peak charging voltage down to 4.2v, you gain substantially more charge/discharge cycles. I would say 4.2v versus 4.3v would result in a 100% increase in charge/discharge cycles.
The other problem is protection circuitry. Most(all?) of the Lithium Ion packs commercially available for the Roombas and Scooba do not have separate balancing/charging circuit boards. They all rely on “protected” cells. The protected cells have a circuit built in to protect them from catastrophic over charging and/or over discharging. These protection circuits are set at the absolute upper and lower limits of the cell’s operational ranges. They are set to protect against explosion and cell instability, not for cell longevity. Here are more specific problems present in each of the iRobot platforms:
500 series – The charging circuit is very touchy and is expecting specific characteristics. If those aren’t met, you will likely get an “error 5” on the screen. Much more likely than you would with a NiMH pack. The internal charger also will consistently charge this pack over-voltage. It’s supposed to be 22.5v on the input side but mine puts out 22.97v even in circuit. I’m taking a wild guess that mine is not the only one that has this problem. By the time you trace the voltage to the battery pack, it calculates out to over 4.3v per cell which is too high.
400 series – The 400 series is probably the best candidate for a lithium-ion battery pack but still not necessarily a good one. Most vendors who sell these lithium-ion packs are relying on an internal cell protection circuit and the Roomba’s over discharge circuit to shut the pack down. Neither of these circuits are actually designed for that purpose. The Roomba’s circuitry is adjusted to optimally discharge NiMH packs and the internal cell protection is worst yet. That is ONLY a last resort and should never be relied on to repetitively be used for that purpose. If the cells somehow become out of balance, this can start happening a lot. The symptom is that the Roomba won’t park or go to a red light, it will simply die in the middle of the floor and become entirely lifeless. The power button won’t do anything until you charge it enough to be recognized. Another problem with the 400 series is that the Li-Ion pack makes the Roomba much lighter and it seems to have a harder time making contact on the dock without the added weight holding it down.
Scooba – This one is one of the worse of all. Personally I have a BAD habit of not taking the battery out right away when the Scooba dies. While I was fixing a Scooba for a friend one time, I was probing around trying to diagnose a problem and found that even when the Scooba is TOTALLY off, it’s still drawing quite a bit of power. I tested the output leg on the 78XX series regulator and found voltage there and a few other places. The logic was all still hot and drawing power as well. The power switch on the Scooba is essentially soft power. This is probably the case with the Roomba too although I haven’t tested it. The difference being the Scooba isn’t designed to dock. Not only that, I’ve always charged the battery out of the unit in the Scooba Charging Base. This is an extra step that I sometimes don’t have time for. As a result, I’ll sometimes forget about it entirely and the battery will sit in the Scooba and over discharge. Doing this a few times will surely fry a lithium ion pack and will eventually fry a NiMH even.
The bottom line is that you are far better off sticking with NiMH battery packs for the Roomba, Scooba and Dirt Dog. I have used the Tenergy Scooba Battery with success in the past although one of my friends has had VERY mixed results with the Tenergy rechargeable AA’s. For the Roomba, they have OEM Roomba batteries at the Renton, WA Frys for $40 which is a steal for a local store in my opinion since the list price is $69.99 or they have them on Amazon even cheaper at this link: iRobot 4905 Battery. For the 500 series, I have had success with this Aftermarket 500 series APS Battery sold by Allergy by Gone. If iRobot products were designed specifically for Li-Ion chemistry, I have no doubt that Li-Ion packs would perform fantastically and we would all benefit from longer run times and cell life but since they are not, I don’t think it’s a great idea to run these types of battery packs in your robots.