Load testing a car battery

Inspired by this thread

I want to make a dummy load for a car battery but I was wondering eg for a typical 60Ah battery how many amps should it be able to provide and for how long?

If for example the battery can provide 20 amps for 30 seconds is this a good indication or 20 amps is not enough load?

And I know there are commercialy available load testers but it’s fun making your own stuff.

The load testers I used back in the day typically loaded the battery down to 10 volts, which depending on the capacity would be 200-300 amps (i.e. roughly half of the rated CCA) for 15 seconds.

Thanks!

Therefore at 200Amps I need a 0.06 Ohms equivalent resistor, capable of dumping 2400 Watts.

I found some heating elements but they are made for 230VAC and they have too big resistance, 12 ohms. I’ll have to put 200 of them in parallel to get 0.06 ohms. Of course then it would be able to handle several hundreds of kilowats, provided I could find a swimming pool to immerse them in water :smiley:

I would caution against hobbyist mucking around with this power level. The voltage is low enough to seem safe, but the phenomenal current level can be dangerous (= lose fingers before you can let go of a white-hot wire) or even deadly.

with an auto battery load tester you want it to be quality made with a good design. store bought models are fully encased (with ventilation) and lots of insulation to prevent unwanted electrical contact. an auto battery will melt metal.

The last time I needed a low-resistance high-power load (to test a power supply, not a battery), I used a long length of wire in a bucket of water, sufficient to get about 0.24 ohms of resistance for 50 amps at 12 volts for example, 14 feet of 22 AWG has 0.234 ohms of resistance at 25C (although resistance will increase to 0.299 ohms if the water starts boiling, but this is fine for a short test, 10-14 feet with easy adjustment for temperature changes would be better for a few minutes or more, but car batteries don’t need to supply current for long).

For a car battery, you’d want to use thicker wire, perhaps two or more strands in parallel for heat dissipation (e.g 2 18 foot strands of 18 AWG for 0.06 ohms; use magnet wire which is electrically insulated but the insulation is so thin it doesn’t impede heat dissipation, make sure the entire length of wire is in the water, with thicker wires connecting to it, note also that load voltage and resistance of connecting wires should be factored in, thus take a foot or two off the length).

A 60Ah battery should be loaded @ 180A for 15 seconds. At which time the battery should be @ 9.6V or above.

car battery testers use carbon piles.

This. It isn’t just that you will be sinking hundreds of amps, but that a mistake or fault could create a situation where you are sinking thousands, or you vaporise something. A car battery can source a prodigious instantaneous current, enough to cause many things to simply melt or vaporise, and if you are too close to it when that happens you can end up with very nasty injuries. Loss of fingers is easy. A horrid injury is vaporising a wedding ring. Nicely cauterises the wound, but it is far from pain free, and the damage caused prevents surgical repair. A tester needs a switch that can handle the current, and not weld itself solid. Overall this is an exercise in engineering for currents much higher than you might really wish to handle in a homemade device.

This, and what NitroPress said that didn’t get quoted here. Flooded lead-acid batteries have poor power densities, but holy fuck they can deliver motherfucking shitloads of current. I have personally witnessed a 12 volt car battery heat a 3/4" combination wrench to the melting point in a single-digit number of seconds.

Of course I plan to make it as foolproof as possible. It will have a button or several buttons for prespecified time of operation (eg 5 10 and 15 seconds) so it will be impossible to let it on indefinately.

I will also probably use a starter motor solenoid as a switch, because I don’t think there’s any other type of switch that can handle so much current.

Well, there are, but you’d have trouble finding one as suitable at any reasonable cost. Good choice. Get one from the heaviest-duty application you can find, like a big-block commercial truck with a cold-weather package.

In addition to the other dangers already mentioned, note that faulty batteries can sometimes explode. They can build up hydrogen from electrolysis of the electrolyte which can explode from any kind of spark. Also, if there is a fault inside the battery, the high levels of current mentioned above can cause the battery to overheat and boil the electrolyte, and the battery can explode from the pressure.

Design your test rig accordingly. Getting sprayed with battery acid and bits of plastic and metal at high velocities is generally considered to be unpleasant by most folks. A couple of small well-placed panels around the battery can insure that none of the exploding bits are directed at you. Locating your switches someplace other than right next to the battery helps to get you out of the way as well.

it is fun to build stuff; though some analysis is useful. both cost and quality are issues.

if it might be used frequently or in a demanding manner then you would want a higher quality build (so it doesn’t break with use). if used for an exacting procedure then is purchased quality higher than you could build (e.g. test equipment is good purchased)?

consumer/hobby import auto battery loads can be purchased for about $50 to $75.

Thanks for the advice. I am thinking of placing the battery inside a box made out of thick plexiglass. There will be a switch so the device cannot be operated with the box lid open. The resistance can be a simple wire immersed in oil/water as Michael63129 said. I will also have a water/oil level monitoring so the device won’t operate if the wire is dry.

I think I should better use a uC, there’s quite some logic involved and then I can add stuff like an LCD screen.

Your load will change as the test progresses. As the voltage in the battery falls during the test you will need to decrease the resistance to maintain a stated load. This is why manual carbon like testers have a variable load knob.
Good luck.

Wait for a sale at Harbor Flotsam and they are a lot cheaper than that. A load tester is the sort of thing I’d use seldom enough that I’d be willing to give a home to such.

Turns out they are on sale now:

Damn you, Red Baron! I just spotted that in my hot-off-the-press HF flyer and came racing to post the information.

I really do realize the fun of doing it yourself - I’ve spent most of a career doing so. But add up the cost of parts to build one, the risks of building it wrong, and the infrequent need for such a beast… and if borrowing AutoZone’s isn’t enough, then $60 out the door for a commercially made one is the next best thing, and all other choices are pretty far down the list.

Build a time machine or a good voltage display for each of your cars* or something instead.
*Seriously.