What resistive load to perform battery capacity test?

[Skylark]

My newly replaced domestic bank (May) does not seem to retain charge. The bank is 4x 110 Ahr. Between visits to the boat, say once every two to three weeks, the resting voltage shows around 12.5 to 12.6v when I turn-on the switches. The other two banks, engine and bow thruster, always show >12.7v despite being 8 years old.

I’d like to do a capacity test. I’m hence looking for a resistive load of around 2.5 ohm, 100W, to give around 5 amp draw.

Ebay shows resistors around this size for around £10. Does anyone have experience of using these and/or an alternative recommendation for a load?

 

[thinwater]

You don't have an ammeter on the panel? Or buy a clamp-on DC ammeter--you will use it. Just turn on enough stuff to get the load you want.

You have the right idea; the only way to measure capacity is load over enough hours to pull the battery down ~ 30% or so. Also remember that voltage and SOC varies with temperature. Voltage means little unless you know the temperature of the battery.

Note that a resistor must be rated for the amps or it will overheat.

 

[starfire]

My newly replaced domestic bank (May) does not seem to retain charge. The bank is 4x 110 Ahr. Between visits to the boat, say once every two to three weeks, the resting voltage shows around 12.5 to 12.6v when I turn-on the switches. The other two banks, engine and bow thruster, always show >12.7v despite being 8 years old.

I’d like to do a capacity test. I’m hence looking for a resistive load of around 2.5 ohm, 100W, to give around 5 amp draw.

Ebay shows resistors around this size for around £10. Does anyone have experience of using these and/or an alternative recommendation for a load?

Use an old car headlight bulb

 

[Poey50]

I use a 180 watt version of one of these to capacity-test my DIY lithium pack. Cheap here - or three times the price on eBay. I test at 10 amps because that is a fairly common load for my boat so 150 watt would also be enough but the 180 runs cooler. It is just a small fan heater, heatsink and recording device. The current is held constant so you get an accurate result.

13.57￡ 12% OFF|150w Digital Battery Capacity Tester Voltmeter Adjustable Constant Current Electronic Load Intelligent Discharge 2.2 Inch Lcm - Integrated Circuits - AliExpress

180 watt version. 36.51￡ 20% OFF|180W adjustable Constant Current Electronic Load 18650 Battery Discharge capacity tester meter 12V24V48V Lead acid lithium 20A|Battery Testers| - AliExpress

 

[Daverw]

I use bulb Holder on a flat board with a 25w light bulb, can add a second if I want more draw. Easy, reliable and cheap.

 

[thinwater]

I use one a 180 watt version of one of these to capacity-test my DIY lithium pack. Cheap here - or three times the price on eBay. I test at 10 amps because that is a fairly common load for my boat so 150 watt would also be enough but 180 runs cooler. It is just a small fan heater, heatsink and recording device. The current is held constant so you get an accurate results.

13.57￡ 12% OFF|150w Digital Battery Capacity Tester Voltmeter Adjustable Constant Current Electronic Load Intelligent Discharge 2.2 Inch Lcm - Integrated Circuits - AliExpress

180 watt version. 36.51￡ 20% OFF|180W adjustable Constant Current Electronic Load 18650 Battery Discharge capacity tester meter 12V24V48V Lead acid lithium 20A|Battery Testers| - AliExpress

A new toy to buy! Neat.

I have used headlights in the past. So long as the voltage does not drop much it works.

 

[GHA]

and/or an alternative recommendation for a load?

not sure it matters much, even with a constant current load without a temperature controlled bath & run the batteries more than once down to 10.5v you're unlikely to get an accurate result to compare with the battery datasheet.
Another option would be get new batteries for your multimeter so there's more chance of getting an accurate voltage from the battery terminals. Don't trust any readings til proven accurate enough.
Turn everything on and see what happens to the voltage over time. Take it from there. Temperature can make a significant difference.

One thing to bear in mind is there is data showing lead acid take a good few cycles to break in & provide the stated power. A new battery has less capacity than one which has been cycled a good few times already.

 

[Poey50]

A new toy to buy! Neat.

I have used headlights in the past. So long as the voltage does not drop much it works.

I should add, in case it's missed, that the unit runs off A/C.

 

[Skylark]

Many thanks for the responses.

I want to measure each battery individually, not as a bank of four, hence I can’t use the boat 12v panel meter voltage and current readings.

Using a car headlamp, 55W bulb isn’t my preferred option. While bulbs are widely available, bulb holders don’t seem to be. This should draw about 4.5 amps and take 12 hours to reach 50% SoC, if the battery is good. I’d prefer 10 amps simply to speed-up the process, hence I’m looking for something 2.5 ohm, >100 W.

I would have thought that taking a battery to 10.5v carries a risk of it not recovering. I was planning to stop the test at 11.9v (which I thought is 0% SoC for a LA battery).

I’ve never read that a new LA battery needs cycling to reach full capacity. Any reading reference to this?

Edit, DC amp clamp meters are pretty expensive and I don’t have one. I was hoping to use calculated current from known resistance?

 

[fredrussell]

For ten amps use two car headlight bulbs. You don’t need bulb holder, with a bit of persuasion normal female blue leccy terminals will fit the male connections of a bog standard h4 bulb.

 

[GHA]

I would have thought that taking a battery to 10.5v carries a risk of it not recovering.

If it's late in life and can't recover from a deep discharge it might be nice to know now rather than later.?

The breaking in cropped up with trojan, Can't remember if that's where I saw it first, though nothing special about trojans so seems a chance it could also apply to other lead acid batts.
https://www.continentalbattery.com/assets/TrojanUserGuide.pdf
page 25
"Trojan’s deep-cycle flooded batteries take between 50 – 100 cycles to achieve full, peak capacity"

 

[PaulRainbow]

If it's late in life and can't recover from a deep discharge it might be nice to know now rather than later.?

The OPs batteries are not late in life, they are "newly replaced". Running them down to 10.5V would be pretty stupid.

Better to run them down to 50% and see what the useable, real World, capacity is.

 

[thinwater]

Many thanks for the responses.

I want to measure each battery individually, not as a bank of four, hence I can’t use the boat 12v panel meter voltage and current readings.

Using a car headlamp, 55W bulb isn’t my preferred option. While bulbs are widely available, bulb holders don’t seem to be. This should draw about 4.5 amps and take 12 hours to reach 50% SoC, if the battery is good. I’d prefer 10 amps simply to speed-up the process, hence I’m looking for something 2.5 ohm, >100 W.

I would have thought that taking a battery to 10.5v carries a risk of it not recovering. I was planning to stop the test at 11.9v (which I thought is 0% SoC for a LA battery).

I’ve never read that a new LA battery needs cycling to reach full capacity. Any reading reference to this?

Edit, DC amp clamp meters are pretty expensive and I don’t have one. I was hoping to use calculated current from known resistance?

Klein has a meter with DC clamp for $65. A good value in a multi-use tool.

You can still use the boat as a load. Just hook the batteries up one at a time. Jumper cables.

 

[KompetentKrew]

Use an old car headlight bulb

Old!? The pound shoppe has them brand new!

 

[William_H]

Use something like this for amp meter. .1 ohm ww resistor 0.1 Ohm 5 Watt Wire Wound Resistor | Jaycar Electronics to measure current using your multimeter on 200mv range (200mv equals 20 amps)
You might like to be adventurous and use old SS rigging wire as a load resistor. Or even a lot of ss lock wire. Wind around something heat proof.
12v down light bulbs are easy to find in various wattages. As intimated don't expect to actually get the rated AH out of the battery. 50% on test would be really good. ol'will