# Battery tester?

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#### MikeMl

##### Well-Known Member
I need to find an inexpensive method of testing the capacity of 12V (or 24V) lead-acid batteries. The batteries have a nominal capacity of ~35Ah, and for the purpose of a capacity test will be discharged at the 1C rate = 35A (or 17.5A for the 24V battery). I can handle the timing, measurement, cutoff limit, and display using an Arduino.

The discharge rate does not have to be constant-current (electronic load). I would settle for discharging the battery into a fixed resistance, which would be approximately R=12.5/35 = 0.37Ω (or 25/17.5 =1.43Ω) . The big problem is that the resistor(s) must be capable of dissipating 12.5*12.5/0.37 = 437W (or 25*25/1.43=437W)! That is serious heatsink with fans territory...

I am looking for ideas on what to use as an inexpensive load for the battery to discharge into.

#### JimB

##### Super Moderator
I am looking for ideas on what to use as an inexpensive load for the battery to discharge into.

JimB

#### N11778

##### Member
110v 750 watt HarborFreight inverter into the bottom element of your hot water heater.
Should be about the correct current. if its higher than you want put an scr or triac controller in the output of the inverter.
I do it now to heat the water from my solar system.

#### Pommie

##### Well-Known Member
Five 100W 1R5 resistors off ebay?

Mike.

#### MikeMl

JimB
A 60W sealed beam is about $15 here.... 110v 750 watt HarborFreight inverter into the bottom element of your hot water heater. Should be about the correct current. if its higher than you want put an scr or triac controller in the output of the inverter. I do it now to heat the water from my solar system. Not inexpensive... Best I have been able to find so far is www.1000bulbs.com or www.bulbamerica.com #### MikeMl ##### Well-Known Member Most Helpful Member Five 100W 1R5 resistors off ebay? Mike. I tried some of these, and they may be ok if I mount a fan to blow on the heatsink, and I de-rate them about 50% #### Pommie ##### Well-Known Member Most Helpful Member I tried some of these, and they may be ok if I mount a fan to blow on the heatsink, and I de-rate them about 50% Use 10 3R ones. They should cost around 1USD each if you can wait a month for delivery. Should be fine in free air - no fan required. Mike. #### MikeMl ##### Well-Known Member Most Helpful Member #### alec_t ##### Well-Known Member Most Helpful Member Electrolysis bath? #### JimB ##### Super Moderator Most Helpful Member Car headlight bulbs? A 60W sealed beam is about$15 here....
Sealed beam?
That expression may have a different meaning for you in the USA, but to me that is rather passé 1960s technology.

$15 ? I wasn't suggesting that you buy new ones, how about a visit to a junk yard full of old cars? Surely you could get a handful for$10 ?

JimB

#### Nigel Goodwin

##### Super Moderator
I would agree with car bulbs, I've got an old front fog light I've used as a dummy load for many years.

I would also like to point out that a 35AH battery is unlikely to be able to provide 35A for an hour, that's not how AH ratings work - it's measured at a specific load, over a considerably longer time.

#### MikeMl

##### Well-Known Member
I would agree with car bulbs, I've got an old front fog light I've used as a dummy load for many years.

I would also like to point out that a 35AH battery is unlikely to be able to provide 35A for an hour, that's not how AH ratings work - it's measured at a specific load, over a considerably longer time.
During the test, the battery will be discharged to 11.0V at an initial discharge current of ~35A. The current will decrease a bit as the battery terminal voltage decreases. I plan to integrate the current ∫di/dt to determine the battery capacity. The tester will report the capacity of the battery in Ah at the nominal 1C rate, and how long it took to reach 11V. The discharge time will likely be less than 30min.

The fog light is likely less than 100W. I need five times that..., plus I need series/parallel switching to test either 12V or 24V batteries.

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#### jpanhalt

##### Well-Known Member
Hi Mike,

This is "perfect" : https://phoenix.craigslist.org/evl/bpo/d/phleger-m15-electric-trolling/6395783131.html
Of course, you will need a barrel of water, which might cost more than the motor where you live.

Minn Kota is another popular brand. They also make variable speed ones. If you can't find the perfect one at a reasonable price, a little work on the prop should suffice to adjust current -- just like putting a club prop on an airplane. From what I read, something with 20 to 30# thrust should have close enough current for adjusting.

#### MikeMl

##### Well-Known Member
Electrolysis bath?
Or maybe boiling off distilled water?

A 12V battery supplying 35A for a half hour will deliver about 420W*0.5h = 210Wh=756kWs.

I calculate that boiling off (converting to steam) about 100g of 20degC H2O requires 260kJ = 260kWs
so 100gH2O/260kWs*756kWs = 291gH2O

So if I bolt the resistors to a stainless steel container, pour in ~300g of Distilled Water, that will discharge the battery before the water boils away...

#### Nigel Goodwin

##### Super Moderator
The fog light is likely less than 100W. I need five times that..., plus I need series/parallel switching to test either 12V or 24V batteries.
It's less than 100W, but you simply use more of them - and that gives you your voltage switching as well.

Have you considered electric heater elements? - obviously they get red hot, but are designed to do so. As it's just a coiled length of resistance wire you can uncoil it, cut it to suitable lengths, and then series/parallel the lengths accordingly. The main problem is likely to be to try and do it in a safe fashion, to prevent burns.

#### debe

##### Active Member
This is my battery load for testing capacity 55W to 230W. Basicly free from the scrap yard 55/60W head light globes & bases, use as many as you require.

#### dr pepper

##### Well-Known Member
Amongst some of the junk I have to maintain at work theres some reel movers, these things move 6 ton reels of paper, they have traction batteries, 12v each at 10ah, and can be discharged in the region of 100 amps for a few seconds.
You can get high discharge smallish capacity lead acids.
I use bulbs as dummy loads too, the drawback though is the resistance is much lower when cold, esp with halogen.

#### MikeMl

##### Well-Known Member
If I were to use MR16 12V 50W Halogens, the inrush current looks scary. The ratio of cold resistance to hot resistance seems to be ~0.08, meaning that since I plan to switch them with an N-FET, it will have to accommodate an inrush current of hundreds of amps...

Even using PWM would still have the same peak current, unless I use an inductor?

Any ideas?