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Got a car, motorcycle, boat or mower that sits for months? The Black+Decker BM3B battery maintainer keeps it charged for about $30.
I bought the BM3B to keep my Plymouth Prowler’s battery alive through long Chicago winters. Here’s what comes in the box, which leads to use, how to hook it up, and what the lights mean.
- Cheap compared to $300 advanced maintainers
- Trusted brand
- 6V and 12V
- Long cords to reach the wall
- Simple status lights
- Lower-end unit, not a full battery-health analyzer
- The O-ring quick-connect leaves a wire under the hood
▶ Watch the full review on YouTube
Do you need a battery maintainer?
If a car sits parked for a while, like a car you store for winter, or a motorcycle, boat or even a lawn mower, a maintainer keeps the battery from going dead.
You can spend a lot on maintainers that monitor battery health and cycle the charge. This is a pretty low-end one, but it had good reviews and it’s all I need, at about $30 instead of $300.
What comes in the box
The unit is compact and light, with a long cord to the wall and a long cord to the battery side, so it can sit on the floor or a workbench.
You get three ways to connect: alligator clamps, O-ring leads you can wire to your battery terminals for a quick-connect plug that stays under the hood, and a 12V accessory-port plug. The clamps are what I use; to me they’re just as easy as the quick-connect.
How to hook up the BM3B
- Set the switch to 12V for cars and most 12V systems (6V is for things like some motorcycles or older cars).
- Connect the clamps to the battery first, before plugging into the wall, so the unit can sense polarity.
- Clamp the positive terminal.
- Connect the negative. My instructions said to ground it to the body, so I did that on the Prowler.
- Plug it into the outlet. Never touch the clamps together.
What the lights mean
Red means something’s wrong. Flashing green means it’s charging. Solid green means it’s fully charged. Mine was set to 12 volts and flashing green, so it was charging. As long as it keeps my battery from going dead through long Chicago winters, I’ll be happy.
FAQ
Is the BM3B 6V or 12V?
Both; there’s a switch.
Should I connect it to the battery or the wall first?
Connect to the battery first, then plug it in.
What does the flashing green light mean?
It’s charging. Solid green means fully charged.
Can I leave it connected all winter?
That’s what I bought it for: keeping a stored car’s battery from going dead.
Which leads should I use?
I use the alligator clamps. The O-ring leads are a quick-connect option.
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