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Batteries wired in serie

I have a rechargeable battery mower with two 12 volt batteries wired in series to produce 24 volts. I installed new batteries at the beginning of the season. The power seems to have dropped off and this is indicated by the meter on the mower. I checked the batteries after a full charge. Across both batteries I get 24.4 volts. However the voltage of one battery is 13.3 volts and the other is 11.1 volts. I am debating whether to replace just the weak battery or replace both batteries.

Can you charge the weak battery by itself?

This late in the season if you can cover the yard on one charge I would wait until just before mowing season starts again. Or as my neighbor does with his cordless trimmer he has to do the front one day charge and do the back the next. He plans to buy two batteries in Feb.

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The batteries should be matched. I agree, wait till next season then put in two new ones.

Are these lead acid batteries? That type should be 12.6 volts with the charger not connected, not 13.3.

@texases. I have thought about charging the weak battery with my 12 volt charger and see if it will hold a charge. It is a little inconvenient because I have to remove the cover over the motor and batteries, but it is worth a shot. The pair of batteries cost $70, and the batteries were new last spring. I had hoped to get several years from the batteries. I will try charging just the weaker battery.
I can probably fimish the season with the present batteries. However, for $70, I could buy seven years worth of gsoline and oil for a gasoline powered mower. I bought the mower used as an experiment to see how well a rechargeable battery mower would work for me.

The one dead battery ie 11.4 v can kill both batteries, At min replace the low voltage battery, Keep an eye on both and go from there?


Not knowing what mower we are talking about don’t the batteries just pull out and plug in to a charger ? Maybe the charger is not doing it’s job

@BillRussell. The batteries are sealed lead acid batteries. The mower is still operable.
@VOLVO_V70 I will probably wait until next year to replace the batteries. I do have a backup gasoline mower if I don’t make it through the season. As I stated earlier, I bought the mower as an experiment to see how well it would work for me.(I’ve been tempted to buy a hybrid or EV for the same reason)

@VOLVO_V70. The mower is a Black and Decker CMM 1200. The top cover has to be removed to access the batteries. It only involves removing four screws. The newer Black and Decker mowers do have a battery pack that is easy to pull out. On my mower, there is an LED that glows red when the batteries are being recharged and then glows green when the batteries are fully charged.

Batteries have internal resistance. Not done on purpose, just a physical limit due to the way they are constructed. The lower it is, the more power the battery can deliver. The one battery measuring 11 volts probably has 10 times or more internal resistance than the other good one. Since they are in series, the 11 volts battery will limit how much power the battery pair can deliver to the motor. If you replace the bad one, that should return you to like-new battery resistance for the pair. Whether it makes $$$-sense to buy and install a new battery, or buy a gasoline lawnmower, my guess you’d be better off $$$-wise w/a gasoline lawnmower. It’s pretty common where I live to find used lawn mowers as give-a-ways placed by the side of the road. Usually just the blade needs to be balanced and the carb cleaned out is all, then they work fine. Talking about the inexpensive push-style rotary gas-powered kind, that you push to move them across the lawn, but the blade is turned by the engine.

George, I disagree with you in this case. Using the (somewhat simplistic) model of a perfect battery in series with a resistor, a high resistance in one unit of the pair would not change the no-load voltage. They both would read 12.6 volts. Only under load would you see a difference.

The low no-load voltage usually indicates a bad cell. Which translates to a lower no-load voltage for that cell and a higher resistance.

The lead acid 12V are, internally, 6 cells in series, each a nominal 2V, but actually about 2.1 to 2.2V just off charge (or on trickle charge) and about 2.3 at a standard (not fast) charge. That 13.3V is in the ballpark, but the 11.1 screams to me that a cell shorted internally. This is a fairly common failure mode.

Trying to charge it separately will not likely do any good, and may be dangerous if the charger doesn’t limit current appropriately for the battery.

What can happen with (any type of) batteries in series is if one battery is in a higher state of charge then the other it will fill first and have a higher voltage than the other(s) then overcharge.
This is more likely to happen with new batteries, even if they have identical capacities and internal resistance.
I have quite a bit of experience with electric vehicles using lead-acid batteries in series.
The thing to do with new batteries is to fully charge each battery individually before putting them in service.
Since flooded lead-acid batteries can tolerate some overcharge they can reach a state of balance if they aren’t too far apart.
Still it’s good practice to check individual battery voltages periodically to see if they’re getting out of balance.

In the case of lithium-ion batteries, which cannot tolerate overcharging, control circuitry is needed to actively keep series cell voltages in balance.
There is a device commercially available to actively balance series lead-acid batteries, but the name escapes me.

Thank you everyone for your comments. I hope to make it through the season with the mower. If not, I have a backup mower to mulch the leaves. It burns a lot of oil, but it still runs. I may have damaged the batteries early in the season. I have a friend who is suffering from cancer and Parkinson’s disease. His wife was reluctant to ask for help, but two of us took our mowers to their place and went to work. The grass has gotten quite long ans the overload circuit breaker kicked off the mower a couple of times. I really ran the batteries down. I probably should have taken my gasoline mower to do the job. I do like the job the battery powered mower does. I bought it used from a friend who decides to hire someone to do her mowing. Since I can still get through my yard with the batteries in the mower, I’ll wait until spring and replace both batteries. Thank you all for your help.

Try this now: switch battery position before you recharge them next and see if the voltage is the same as before. The lower charged battery will probably still be the lowest, but it is no cost test.

@jtsanders. Thanks for the suggestion. I tried that but it didn’t do any good. I will ultimately have to replace the battery or both batteries.

You still have sufficient power to use the mover. So what that one battery is a touch below the anticipated voltage. Just keep using it until the thing falls apart and then worry about batteries. I do not see a problem here.

I suggest you fully charge (like overnight) each battery separately then see how they behave.

The thing I like most about battery powered equipment is the battery runs out in an hour or two and so I’m forced to give up for the day. Ah shucks, the battery ran out of juice. Yipee! It’s Miller time!

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@GeorgeSanJose. I really dislike mowing and my wife loves to mow. One would think that would be a perfect scenario. Unfortunately, Mrs Triedaq has had two foot surgeries and two rotator cuff surgeries, so I won’t let her touch the lawnmower. I can get through my entire yard on one charge, so for me the rechargeable battery mower fills the bill. I have a rechargeable battery powered string trimmer which works great for me. I have a friend who plays tuba in our band and we share rides to rehearsals. He is trying to talk me into buying an electric assist bicycle. One still has to pedal, but the electric assist makes it easier. He told me coming back from rehearsal this evening that he rode the bike 32 miles today. Most days when it doesn’t rain he rides 10-30 miles and he is 80 years old. I really believe the future is in rechargeable battery equipment.