Once I put it away the battery had dropped to 10.5 again. It’s back on the tender but it’s my other one that doesn’t cycle the charge.
So will the battery tender bring it back up over time or do I need to put it on a proper charger or need a new battery?
The no-load voltage of a lead acid is important. In this case, 10.5V is a good thing. The minimum discharge voltage for lead acid is 1.75V per cell, where 6 cells x 1.75 = 10.5V. So you probably have a battery that can be recharged and perform well afterwards.
Before charging will take place, a charger has to overcome the internal resistance of the battery. The internal resistance of a battery is what causes it to self-discharge over time - it is as if there were a small resistor directly across the terminals of the battery. A float/trickle/tender-type charger typically provides just enough energy to match the self-discharge rate. This is why you can usually leave a no-brain-box 500mA charger on a car battery forever without over-charging it or boiling the electrolyte off ... the charger output accounts for the self-discharge rate and no more. Also note that small-output chargers can take a very long time to fully charge a completely discharged battery. If you are putting a net 2A per hour (that takes more than 2A output!) into a battery that needs 400A to be charged, it will take 2A x 24hrs x 8.33 days to fully charge it.
So the answer to your question is yes, put the battery on a big charger - one that puts out at least a couple of amps (I usually charge at about 10A for a car battery) and charge it to 100% before you try to maintain it with a trickle charger. A big charger should also have a beneficial current profile where it is high for the bulk of charging then tapers off as you approach maximum voltage. This will not only charge your battery reasonably quickly, it will also bring the charged battery capacity very close to 100% before reaching max voltage.
Note that depending on conditions and a particular vehicle’s system, it can take a long time to charge a severely discharged battery while driving. Automotive charging systems are typically dumb - they just keep dumping energy into the battery. So the usual alternator just keeps up with the electrical loads and dumps a little extra into the battery to account for the few seconds it took to crank the starter. If the alternator overcharges your battery, it will boil electrolyte off - you’ll notice residue around the vent caps and have to add distilled water occasionally (my Toyota 4Runner does this).
It is especially important to recognize what’s happening in severe conditions. In Alaska, I would occasionally have a hard-start session where it might take two or three minutes of intermittent cranking to start the engine at -30įF or -40įF. Not only was the battery capacity severely reduced at that temperature, but it’s ability to charge was also severely reduced. So the drive to wherever I was going after the hard start would never restore the energy I lost during starting. The solution was to wait until the battery came up to room temperature in the garage, then put a big charger on it. Failure to do this was a sure way to experience a dead battery a few days later (especially at -30įF).
Two more things ...
- A charged battery has lead on the cathode and lead dioxide on the anode. Discharging the battery chemically converts the surface of the plates into lead sulfate. Over time, some
of this material drops off the plate into a well below: the longer the battery sits in a discharged state, the more lead sulfate will drop off. Any material that drops off is not available to be converted back into the original material during charging ... so the overall capacity of the battery is reduced. As a lead acid battery ages and more and more lead sulfate drops off the plates, the well fills up and the lead sulfate will eventually make a circuit across the bottoms of the cells and short them. This marks the end of the battery’s life.
- All batteries have some internal resistance. The higher the internal resistance, the higher the self-discharge rate. The internal resistance of lead acid, nickel-cadmium and nickel metal hydride batteries are all fairly high in comparison to a lithium ion battery. The low internal resistance of a LiFePo4 battery is what lets it sit (at no load) for 8 - 12 months before it needs charging. A healthy lead acid battery might only last 2 months before it needs to be recharged.
p.s. - Number of times I’ve used a load tester on my lead acid batteries ... zero. If you know the history of the battery, you’ll have all the information you need to make a replacement decision.