Dead Battery issues

EOD Guy · Jul 29, 2025

I haven't had a single issue with mine. I do know that if they ever go flat, sometimes you have to fool the charger by sistering a charged battery to the flat battery until the charger starts charging etc..... but mine hasn't discharged to a point of no start since I installed it.

ultane · Jul 29, 2025

AgRN16 said:
Yes, usually they do. I have a 5th gen 4runner and put i an AGM. I had to make a mild mod to the fuse box so the alternator would pass through a higher current to keep the battery properly charged up.

Can you tell us how you did that? Pretty please??

D-mac · Jul 30, 2025

Grandpawmoses said:
Don't AGM batteries require adjusting the charging voltage of the vehicle? Maybe I'm thinking of something else though.

The manufacturers design aftermarket AGM batteries to be maintained with stock/OEM vehicle charging systems, and have worked great this way for many years. Consult a battery manufacturer for more detailed information.

basicxj · Jul 30, 2025

D-mac said:
The manufacturers design aftermarket AGM batteries to be maintained with stock/OEM vehicle charging systems, and have worked great this way for many years. Consult a battery manufacturer for more detailed information.

Certainly some AGMs do, but some of the more high performance AGM deep cycle options need to be topped off with an AGM compatible charger periodically to maintain an optimal state of charge.

That was true with both a Northstar and an Odyssey I ran in a previous vehicle. I'd get 13.9-14.7V at the battery terminals for charging voltage depending on temperature and RPM, but that wouldn't get the resting voltage of the battery up to the 13.0+ V they were capable of when topped off with a charger. Some AGM deep cycle batteries need a little punishment to take a full charge. Consulting the battery manufacturer is good advice, best to read up on specific batteries before buying if you plan on letting the vehicle do all of the battery maintenance for you.

AgRN16 · Jul 30, 2025

ultane said:
Can you tell us how you did that? Pretty please??

Voltage Booster PRO "The O.G." (5th Gen 4Runner & GX460)

VoltageBoosterPRO is a user adjustable, plug and play, diode modification with an inline fuse so that you can increase your alternator voltage to optimize your AGM battery charging.

voltageboosterpro.com

To be clear, this is vehicle specific to 5th gen 4runner and GX460.

D-mac · Jul 30, 2025

basicxj said:
Certainly some AGMs do, but some of the more high performance AGM deep cycle options need to be topped off with an AGM compatible charger periodically to maintain an optimal state of charge.

Every AGM battery manufacturer's guidelines I have seen only recommends a boost/charge if/when the battery is deeply discharged below around 12.5v, which does not normally happen on a daily driver. Per manufacturers, regular charging beyond 13V is not recommended for longest battery life. But maybe there are some batteries that can charged to the maximum 14.5V+ even though the batteries are supposed to support a 12V system for some reason.

EOD Guy · Jul 30, 2025

All well and good except the LC doesn't have an alternator.................

basicxj · Jul 30, 2025

D-mac said:
Every AGM battery manufacturer's guidelines I have seen only recommends a boost/charge if/when the battery is deeply discharged below around 12.5v, which does not normally happen on a daily driver. Per manufacturers, regular charging beyond 13V is not recommended for longest battery life. But maybe there are some batteries that can charged to the maximum 14.5V+ even though the batteries are supposed to support a 12V system for some reason.

A normal flooded lead acid battery would have a resting voltage of 12.6V with optimal state of charge. The average vehicle should have no trouble maintaining that resting voltage.

Based on design, these two AGM deep cycle batteries I mentioned are capable of a higher resting voltage- an overnight top up on my NoCo AGM charger (done about every four weeks) would result in a 13.0V resting voltage measured at the vehicle when the charger automatically went into maintain mode at completion of charging. 13V wasn’t a target state of charge in manual mode on the charger, it was simply the resting voltage achieved at full state of charge based on the battery capacity.

My vehicle had some constant parasitic drains, in the form of a dash cam setup with motion and impact activated recording, which was triggered pretty much constantly when parked at work- heavy vehicles passing by all day five days per week. That would see resting voltage down to as low as 11.3V after several weeks. Most computerized vehicles won’t start the vehicle when discharged to or below 12V.

Those premium AGM deep cycle batteries made sense for my usage, as they’d still have enough reserve to start the vehicle at 11V or so and sustained no damage from regular discharge. I will probably add a similar camera system to the Land Cruiser, complete with an upgraded AGM deep cycle battery.