# Choosing a battery: is a harbor freight solar battery OK for a R/C Lawnmower?

I have built an R/C Lawnmower. I call it the Honey Badger, because it tears stuff up (that's a good thing). Well, I used used batteries to get the project going and now it's long past time to get the Honey Badger going again.

The Honey Badger is built on an electric wheelchair frame, and originally used wheelchair deepcycle batteries. U1 if I recall. There are 4 of them wired in 2 banks in series and parallel to give 24V for the 24V motors.

Going down to the used wheelchair parts place is about an hour drive and requires a weekend visit and will get me used batteries of unknown condition.

Contrast that with Harbor Freight, which is 20 minutes away and has solar batteries the same physical dimensions and comparable (?) electrical characteristics. I think with coupons, tax, and after playing the game, I can get a battery for ~$50, about the same price as a used U1. I found that Amazon also has U1 batteries, and they can be had for ~\$120 for 2 with shipping.

Batteries plus will sell me some deepcycle auto batteries of greater Ah capacity for ~$100 each. Gross for each solution winds up being around the same: ~\$240 - ~\$300. Is there a difference in technology between a "solar battery" and a "wheelchair battery"? Is that difference substantial? Given that I'm pretty rough with this thing, is any particular technology any better suited to these tasks? Is there a benefit or drawback to using an automotive battery? I have the charger from the original wheelchair and if I recall, it's good for the capacity and has room to spare. I think it can put out 5 amps. ## 2 Answers First let me state that how long it takes you to drive to a particular location or whether the batteries are used, etc. is all too specific to your scenario to matter with regards to what appears to be your root question, which is, "Is there a difference in technology between a "solar battery" and a "wheelchair battery"? In short, for what you have provided, the answer is no, there is no difference. The wheelchair batteries are lead-acid, as are the "solar batteries" and the batteries at your local battery shop. Lead-acid batteries all charge at about the same voltage (14.7V), are all "dead" at about the same voltage (10.8V), etc. The specifics of each battery, such as the number of charge cycles you could expect, depth of discharge before you damage the battery, etc., would probably vary by manufacturer, but that data isn't typically published and you would have to contact the manufacturer to get it. If I were you, I would use past experience with operational life (in minutes) that you got from your current battery and use that as a guide to spec the battery capacity for your next battery. Ignore driving times, because if you're purchasing the correct battery you should only have to get one. Also, I wouldn't buy a used lead-acid battery. They're easily abused and, just because it can be charged to full capacity doesn't mean that it will hold that capacity. Damaged lead-acid batteries form internal bridges that short out the battery internally; this is what happens with a battery that won't "hold" a charge. Generally, try to prevent ever discharging past 20% of capacity. So, for the sake of an example, if your old battery was 20Ah, and you would run it until it was completely dead (open circuit voltage of 10.8V) to get your lawn dead, then: 1. It took 20Ah to mow your lawn. 2. You were damaging the battery discharging 100% of the capacity. So, you should look at a battery where 20Ah is at least 80% of the capacity, meaning you need a battery of at least (20/0.8) = 25Ah capacity. Anything larger than this may be a waste of money, but anything under this capacity will very likely suffer a significantly shorter life expectancy due to over-discharging the battery. • Regular car batteries are engineered to provide bursts of energy to start the car and accept charge from an alternator. Deep cycle aka marine batteries are engineered to provide lasting power. Which is something the regular car battery cannot do. These facts are implied by my question because I asdumed them to be common knowledge. You answered with textboom science, mostly chemistry. Please elaborate where manufacturing convention might jump in to bite me in the a$$and if I could wind up buying a "wrong type" of battery, if such a type exists. Mar 1 '16 at 22:49 • @allanonmage - I agree; your answer never states that you are seeking to purchase automotive batteries, so I didn't address them. Automotive batteries shouldn't be discharged beyond maybe 10 or 20%. Solar batteries, marine, wheelchair, etc. are all deep cycle batteries. What I've given is general guidance regarding deep cycle batteries - where this will bite you is if you assume all deep cycle batteries are made the same. You need to contact the manufacturer to get life expectancy vs. depth of discharge. And again, don't buy used batteries; there's no easy way to check "how" used they are. Mar 2 '16 at 14:11 • Typically there's a "knee" on the cycles-depth of discharge curve at which point the life expectancy drops dramatically; this point will vary by manufacturer. All manufacturers want to make a buck, so they may advertise a "deep cycle" battery that can be damaged if you discharge it below 45 or 50%, others may be fine down to the 20% I mention above. There's no legal definition of "deep cycle", so if you really care about life expectancy you need to do the proper research and spec your battery to operate within its design criteria, which again is typically not published. Mar 2 '16 at 14:21 Personally, I'd suggest a different approach as I have done. Look into using 18650 cells instead. You can easily score 6-12 of these cells in old laptop batteries, they are lighter and more energy dense than the lead acid batteries you are using and hold up better. You can buy them for approximately \$2 each on eBay with a 5000 mAh rating as well but I find that in old laptop batteries, the charge controller board goes bad before the cells do, most of the time. These cells are lithium ion and you should invest the extra buck or two for low voltage cut off protection, as dipping below the 3.6 volts per cell can cause damage (i.e. the exploding hoverboard incidents).

At full charge they run at 4.2 V and you can easily get clips to hold them and cheap circuitry which will allow you to charge them via standard USB micro cords you no doubt have for your cellphone. Just multiply your volts per cell to equal the volts you need for your series then stack in parallel for more amps. I have replaced powerwheel batteries with a 3S3P pack quite nicely. Personally I use a smart universal charger but there are several ways to go here.