One thing to mention is that 50A is a very high discharge rate to test a single battery capacity. Many batteries are rated at the 10hr rate, which means for a typical 100Ah battery, you should do a discharge at 10A, not 50A. Others are rated at 20hr, which would only be 5A.
If you want to do this scientifically, I highly reccomand the CBA IV, at http://www.westmountainradio.com/cba.php . At 12V it should handle at least a 10A discharge rate. It will do the entire discharge cycle for you and give you a nice chart. The procedure to do this is to charge the battery fully (including rest cycles to make sure the charge has fully equalized through the chemistry, especially if the battery is quite discharged), and then discharge at the rated rate to the voltage listed in the datasheet for the battery. For instance, picking just a random battery datasheet off the web, http://www.solageo.com/ProductResourceFiles/GGRYHXBJGKRX.pdf , shows Capacity at "100Ah @10hr-rate to 1.80V per cel @25C". Deviating from those specs will result in a different Amp hour rating. Generally the higher the discharge rate, the lower the capacity. Also the lower the temperature the lower the capacity. On that same datasheet, you'll note a 'discharge characteristic curve'. If for instance, you were discharging at the 1C rate (which means 1x the capacity of the battery in AH, or 100A), you'd only get around 40 minutes of runtime to a terminal voltage of around 9.6V, which is only 60Ah (1A * 0.6 hours). There's also a capacity factor chart for temperature, note that at 20C (Room temperature), you only get 98% of the rated capacity, which is close enough to generally not matter. Back on the CBA IV, I've occasionally needed to test batteries which are rated at a higher discharge rate than the CBA IV would handle without the expensive add-on 'amplifier'. In these cases, I've opted to discharge at a slower rate and adjust accordingly. Many datasheets will actually give terminal voltages and expected total capacity at different discharge rates, and I've been lucky in that every time I've had to do this the datasheet has the expected values, or at least a chart I could extrapolate from. One other random note: The 'discharge rate' game is played by certain manufacturers to make cells seem better than they are. For all practical purposes a 100Ah 10hr battery is probably very similar capacity to a 110Ah 20Hr battery, since at 10Hr discharge rate, the 110Ah battery will likely have a capacity of 100Ah. On Thu, Jan 25, 2018 at 8:49 AM, Adam Moffett <dmmoff...@gmail.com> wrote: > I wanted to rig up a load test for some batteries. > I don't have a substantial 12V DC load, so I set up a 1000W inverter, a > short extension cord, a Kill-a-Watt meter, and a heat gun. > > With the heat gun on low, The kill-a-watt reads 110v, 606 VA > <https://maps.google.com/?q=110v,+606+VA&entry=gmail&source=g>, and 355W. > > The question is how much load is this putting on the battery? Somewhere > between 30 and 60amp I guess, and either way my multimeter can't measure > more than 10A DC current, so I can't measure it directly. > > My Googling on the topic has failed to enlighten me. My instinct is to > think that Watts is Watts, so I should probably use 355W in my calculation > of battery capacity, but I'm not sure. > > > -- *Forrest Christian* *CEO**, PacketFlux Technologies, Inc.* Tel: 406-449-3345 | Address: 3577 Countryside Road, Helena, MT 59602 forre...@imach.com | http://www.packetflux.com <http://www.linkedin.com/in/fwchristian> <http://facebook.com/packetflux> <http://twitter.com/@packetflux>
