Hi everyone. I recently left on a 3 month trip and left the 4Runner on a trickle charger. When I came back and drove it around I noticed the battery wouldn’t hold a charge anymore (not surprised, it’s from 2016). Got a new battery and it was running fine until randomly it wouldn’t start. I checked my scanner and it showed the battery voltage was super low, around 10.5. Eventually it started and went up to a normal voltage, and then when I turned it off dropped to a normal voltage of about 12.8. Now when I turn it off it’s almost like I’m spinning a wheel of whether the voltage will stay normal or instantly drop super low and the truck barely starts. Anyone experienced this before?

Have you had the alternator tested?
Have you tested (or had tested) the battery?

If you really mean a trickle charger, one that never shuts off but constantly supplies a low voltage to the battery, then you're probably boiling the water out of the battery. You should either get a battery tender that shuts off completely when the battery is fully charged or use the trickle charger on a timer that comes on in the AM and again PM for about 20-30 minutes each time. That will simulate your daily commute to work.

If the battery is not holding a charge then it has a bad cell or cells. If you do not have a tester then take it to a local parts store and have them test it.

I misspoke, it is a battery tender. ThatÂ’s not the issue though as I replaced the battery since it was time to do so anyway. I dont think there is an issue with the alternator since when the car is running I can see the battery is charging and maintaining level through my obd scanner. The issue is that sometimes when I turn it off I can see the battery level instantly drop below a normal amount, and other times it stays at a normal level. Maybe I got a faulty battery.

I have bought bad batteries before, ones with a bad cell. Make sure the battery you're purchasing has a manufacture date within 3 months. They normally don't ship batteries with a full charge, so charge it up before you put it in the car. Wait 3-4 hours and check the voltage. It should be 12.5-12.7 depending on the temperature. What brand of battery is the new one?

If the battery is not maintenance free you can check the battery level with a hydrometer. This will let you check each individual cell's charge and let you know if you have a bad cell. You can also check the water level and make sure the plates aren't exposed.

Another useful tool is a battery load tester. I bought one from Harbor Freight and it will tell you how strong your battery is under load, not just sitting there with a charge. You can't check cell strength with a maintenance free battery so the load tester is a valuable tool.

I have never seen a lead-acid battery drop from 12.7 to 10.5 in such a short time that wasn't faulty. If this is happening consistently then I'd definitely say you have a bad battery or a bad gauge. Use a different volt meter to check the battery charge. The Harbor Freight device will do this in addition to checking it under load. I also recommend using a timer in conjunction with a battery tender if you aren't going to be there to do a daily check on the battery status.

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Another thought, have you checked all the possible sources of battery drains, such as a map or dome light being left on? You might want to put your multimeter in the circuit with the motor off to see what amperage is being drawn. Incandescent lamps can quickly draw down a battery.