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It will depend on the make and type of your batteries.

Once you've been running it for a while you'll get to know when it starts to get sluggish, which is your cue to recharge or load it up.

Adrian

Hi Roy,

lead acid batteries unfortunately have no fix correlation between voltage and remaining capacity, because the voltage is also dependent from load current. Maybe a battery shows 24 volts and ist nearly empty, when measured without current, and another battery has 20 volts and half its capacity remaining, while it is measured under heavy load.

A reliable method to evaluate the battery level of a lead acid battery is to measure the consumed capacity since last fully charging with telemetry. If your battery hat 40 Ah and you consumed 20 Ah, you may be absolutely sure that the tank will reach the base. Why leaving 20 Ah? The nominal capacity of a lead acid battery is measured at 10 hours or 20 hours of discharge. In a tank it will be discharged within 2 to 4 hours. At this quick discharge no lead acid battery reaches its nominal capacity. But the heavier reason for leaving half the capacity is the fact, that the depth of discharge (DoD) should not be more than 50 % when using a lead acid battery for reaching its optimal life time. This value may be exceeded, depending on shortening of life time. Therefore there exists a certain margin for additional loads at an expense of lifetime lacks.

A LiFe battery can endure nearly 100 % DoD without lifetime lacks, when the monitoring of each single cell (!) is good enough. But here is NO MARGIN for additional loads when using the manufacturer's limits. If you want a margin for additional loads, you have to use a warning voltage value some above the absolute limit. LiFes have a fixed correlation between voltage and charging state, so you may avoid measuring of the consumed capacity.

A LiFe battery with BMS may 'final stop' without a visible reason, e.g. when the load current exceeds the BMS limit for an extremely short time.This results in a widely safe use of the battery, but also a 'dead' model in the field. It can only be reactivated by a short charging.

Hi Gerhard,

Thank you for explaining one and other, I really appreciate it.
I was thinking that probably the battery voltage without current could be an indicator but there I am wrong.
Since I already purchased my batteries (lead acid, 12V, 22ah, rec22, Yuasa) I'll stick with those for now, but the benefits you explain are very clear.
Monitoring the consumed capacity is definitely the way to go. I plan on using the Spektrum SPMA9605 for that.

Thanks for the help

Would it be practical to have an 'emergency' battery pack of any type that you could in the event of a battery depletion that you could substitute for the regular main just to enable you to motor back to 'base', your car or charging station? I'm thinking LiPo pack to be used just long enough to save you from having to lug 200+ pounds of dead tank back home.

Jerry

Hi Jerry,

Well yes i would say it is a good idea. If something does go wrong with the capacity monitoring or the batteries them selfs, then being able to externally connect a spare pack will save the day.

And for external use a LiPo is fine. I will give it some more thought

Having had to get a SdKfz 7 across 40 meters and into the car when batteries depleted, 2 spare charged AGMs and a way to connect them would have saved me a whole lot of effort.
I will not make the same mistake again.

Although my 2010 electronics has never leaked a single electron, I figured it might be a good idea to protect the battery and feed cable from shorts with a fuse. It works as a shut off switch as well, as it can be triggered manually. I would have preferred it to be mounted right ON the battery terminal, but a short length of wire was necessary to connect it.

Hello chaps, on a lighter note, I have a long cable winch, bolted to my trailer.....recovered my Panther with no stress..... Mick