I need to do a C20 test on an 860AH battery bank.
I am thinking to use my electric range cooktops that are powered through an inverter. Since the cooktop burners have rheostat variable temperature control, I am hoping to easily set and maintain the 43A DC required load throughout the test. As long as the bare burners (with no pots on them) will keep a constant current draw and not cycle on and off under some kind of protective thermostat control.
Anybody else have a good means of establishing a fixed but variably adjustable resistive load centered around 500W for such a test?
I am surprised I cannot find a simple 400-500W variable power load tester.
Many thanks for any ideas.
Firstly, I would not do a C-20 test on an entire bank. Select a few batteries and test them individually. If un-wired resting voltages are similar then it is likely okay to
assume the others are similar, especially if it is a parallel bank properly wired. Test the batteries in the bank with the lowest 24 hour resting voltage. If it is a series bank test pairs. Remember a C-20 test is 20 hours so you need a load that can be run
safely for up to 20 hours.
*Charge to 100% (net current under 0.5% Ah capacity at 14.4V +/-)
*Equalize battery, if this can be done (not for GEL and non Lifeline AGM's)
*Allow battery to rest for 24 hours at 75-80F (this is not hugely critical after an equalization)
*Maintain battery case temp of 75-80F (I use a water bath with auto heating element but as a DIY just turn the heat up in one room)
*Apply a
constant DC load of Ah capacity ÷ 20 (eg: 125Ah battery ÷ 20 = 6.25A Load) resistors, bulbs etc. will work.. As voltage falls current goes down so have a way to
add in small increments to maintain as close to C÷20 as you can. Switches and small resistors work well for this but so can light bulbs. If using an inverter current goes up as voltage decays..
*Use an Ah counter to count Ah's removed
*If you don't have an Ah counter use time. A C÷20 load should net 20 hours run time before hitting 10.5V for a battery with full capacity. Anything less than a 20 hour run time is a % of 20 hour capacity in actual capacity.... (eg: 100Ah battery delivers 69.54 Ah's at a 5A load and runs for 13:55)
*As voltage falls try to keep current as close to C÷20 as possible
*Discharge to 10.5V as measured
at the battery terminals with an accurate DVM
*When the battery hits 10.499V terminate load and calculate time or look at the Ah's delivered
*Recharge immediately. Recharging at the 20 hour rate, or a low current close to it, can have
some "reforming" abilities and can restore
some lost capacity.
You can build a home tester inexpensively but it will require manual manipulation of current as voltage falls. A Victron BMV-700 is cheap and can be used as the Ah counter as well as your 10.5V cut off, if you add an external relay and use its relay driver port. You could also buy a spare 50mV shunt and Ah counter wire and just pull the Ah counter off your boat for the testing....
I use rather expensive lab grade 400W/40A DC constant load devices, which have programmable battery testing capabilities. For years I used an Ah counter and resistors, bulbs etc.. I then had Mark Grasser design & build me custom a DC constant load device. It worked but was a PITA to use repeatably/daily but at least I could walk away from the test and not need to constantly adjust current.. I have batteries undergoing testing daily so I need an easy to program and set up tester....
The OP is looking to do a full 20 hour ampere hour capacity test. He's not looking to do a cranking load test, there is a HUGE difference.... A cranking load test, with a carbon pile tester, tells you absolutely nothing about how many amp hours of capacity the battery has left in it at the 20 hour rate..