Looking for a programmable DC load with a Wh cutoff - Page 1
Forum section: electronics / testgear

author: captainscarlet
author_rank: Contributor
timestamp: September 30, 2024, 10:19:37 am
content: Hi,I have an application where I need to discharge batteries to a known capacity.They're 1.05Ah at fully charged, and I need to get them to 75% and 50% state of charge. The discharge voltage curve is very flat so I was hoping there was a DC load that would integrate discharge energy and stop at a particular value.The alternative is to build a battery management solution with coulomb counting, but I don't want to build a custom board if a product already exists.Does anyone know of a product that does this?Pros would be low cost and ease of use as it will likely only be used for this one application.Thanks all.

author: Kean
author_rank: Unknown
timestamp: September 30, 2024, 11:05:40 am
content: Assuming they will start in a fully charged state, you can use a smart DC load at a fixed current for a specific time.Many of the traditional bench DC loads require PC control, or have a complex menu system, but there are some "low cost" devices available with this feature implemented.For example this onehttps://www.aliexpress.com/item/1005007433497815.htmlI've not used one, so I cannot vouch for this particular unit, but the specs look quite good for the price. Probably massive overkill for your needs.

author: Kean
author_rank: Unknown
timestamp: September 30, 2024, 11:28:10 am
content: I forgot to say - how accurate does it need to be? If it is just for preparation for shipment, then I imagine you just need to be below the specified capacity threshold.As I suggested, if they are initially fully charged, and are in similar (i.e. new) condition, the discharge curve should be relatively predictable for 50%, maybe even 80%, of capacity.

author: nctnico
author_rank: Super Contributor
timestamp: September 30, 2024, 01:11:57 pm
content: The Korad KEL2000 series has a battery test mode which can stop when it reaches a certain Ah level during discharging.

author: mtwieg
author_rank: Frequent Contributor
timestamp: September 30, 2024, 02:34:31 pm
content: I use the iTech 85xx series, they have this function (called "Battery mode"). Might be overkill for your purposes though.edit: actually it cuts out at a voltage or amp-hour threshold, not a watt-hour threshold.https://www.itechate.com/en/product/dc-electronic-load/IT8500plus.html

author: captainscarlet
author_rank: Contributor
timestamp: September 30, 2024, 04:14:59 pm
content: Yeah, I was looking at that Aliexpress option, or something similar, a KOCAN 150W DC load, it has the correct energy measurement in Wh, but it looks like it only cuts off at a specific voltage.The accuracy isn't too important, maybe +-3% SOC would be good enough, but if I'm finding that the discharge curve is so flat with LFP cells that the correlation between state of charge and cell voltage isn't exactly uniform cell to cell.

author: tszaboo
author_rank: Super Contributor
timestamp: September 30, 2024, 04:58:32 pm
content: Quote from: nctnico on September 30, 2024, 01:11:57 pmSame with Keithley and Siglent.Battery test mode includes time, voltage and mAh cutoff.

author: 2N3055
author_rank: Super Contributor
timestamp: September 30, 2024, 06:53:37 pm
content: If I can ask: are you sure all the batteries have exactly the same capacity?What are you trying to do?Usually you define cutoff voltage where certain chemistry is considered at state of charge you consider empty (10%, 20%), set test current and wait for load to cut off at that voltage while you are MEASURING how much Ah (mAh) you managed to squeeze from it.That is typical test.With your test, extracting 1Ah from 4 batteries that are nominally 1 Ah, but in reality 1.1, 0.95, 1.2 and 0.91 will result 2 of them to be discharged too deep and other 2 to to 10% and 20% respectivelly.

author: nctnico
author_rank: Super Contributor
timestamp: September 30, 2024, 07:10:03 pm
content: The OP doesn't want to do a full discharge. Only take 25% to 50% out of the batteries. A DC load which can stop at a certain mAh and voltage (just to be safe) is suitable for this purpose.

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 01, 2024, 11:54:10 am
content: I've never seen an electronic load which cuts off at an energy threshold (though such a thing would be simple to implement in firmware). But if you cells have a fairly flat voltage, then the effective difference between a charge cutoff and energy cutoff should be small.Quote from: captainscarlet on September 30, 2024, 04:14:59 pmIf you want to cut off at a specific SoC then you want a charge cutoff, not an energy cutoff (as SoC is defined in terms of charge, not energy).

author: MarkL
author_rank: Unknown
timestamp: October 01, 2024, 01:34:59 pm
content: I think your choices of load are going to be very constrained with the watt-hour cutoff requirement.Consider having an external computer monitor the discharge. It could read the wattage being discharged, integrate over time, and then turn off the load when the value is reached. You would probably want to keep any low-voltage cutoff set in the load in case the computer went out to lunch.If the load does not report watts, I'm sure it would happily give you the voltage and current so the computer could calculate it, and then integrate.

author: Kean
author_rank: Unknown
timestamp: October 04, 2024, 10:15:32 pm
content: As mentioned, due to the pretty flat discharge curve monitoring voltage isn't much help with LFP cells except to avoid over discharge.If you want some reasonable accuracy in SoC, then you'll need to do a cycle - i.e fully charge the battery, run a full discharge to measure capacity in Ah or Wh, and finally recharge while measuring SoC based on the discharge test to determine the desired cutoff point.Unless you have wide tolerance in the cell quality, I expect that measuring Ah will be sufficient and give pretty close estimates of the Wh.