Maast
Compulsive Tinkerer
This is the first and easiest DIY battery tester for lifepo4 battery cells, this is for a SINGLE 3.2V cell ONLY. If you attempt to use a 12V battery you'll melt the load, set your dog on fire and make your kids drop out of school. You have been warned.
Overall cost is about $130 US which is dirt cheap compared to buying a 60A tester.
In this example diagram the cell is a 120AH LFP cell, the charging power supply is a 60A Meanwell HRP-300-3.3, and a 60A load. There is a reason for these values:
Cell amp-hour capacity is usually measured at .5C with C being the capacity in amps. .5C means we'll be pulling .5 of its capacity which in this case is 60 amps and the battery will be exhausted in 2 hours. .25C would be 30 amps, etc. You can use a smaller charger but it makes the test that much longer. As it is a full test cycle takes 4 hours or so.
The power supply is a constant current limited power supply which means it'll reduce its voltage to avoid over-amping itself. If you do not use a current limited power supply it'll just shut itself off, start turning on and off until the overload is cleared, or just permanently die. This Meanwell will actually put out about 70 amps initially but because the cell voltage rises while charging and will drop below 60A fairly quickly. In addition the PS has a range of 2.8-3.8V and will need to be adjusted to 3.65V out of circuit (no load or battery) using the little phillips screw head near the power light. There are lots of meanwell clones out there if you hunt around. Whatever you use it has to be current limited.
The HRP line goes from HRP-75 all the way up to HRP-1000 (75 to 1000 watts) depending on how high you want your charging amps to be and how much you want to spend.
The best price I've found new is on Ebay for $75 including shipping, you can often find them used for a lot less.
The 12V source can be another power supply, or a 12V battery on a trickle charger. The meter only draws 2 watts so you dont need much. The Amp-Hour meter needs it's own separate power supply because LFP cells do not provide enough voltage for it to operate.
This is all assembled on a 24"x36" sheet of 1/2" plywood.
The meter is a PZEM-015 with 300A shunt can be found on Amazon for about $20 US. It comes with some wire but they tend to be a bit short, you'll probably need some extra stranded 18 gauge wire.
The amp-hour meter also displays voltage, current amperage, time watt hours and resistance. The only thing we're concerned with is the voltage, amperage and amp-hours. I prefer to use the shunt vs the hall effect amp hour meters because they're more consistant and accurate.
The 150A Fuse/Switches can again be found on Amazon for $16 but less expensive fuse/switches can be substituted, I just liked the convenience of the connections. Keep in mind these types of fuses' amperage are often badly overrated, in my tester I'm using the 300A versions.
All the main current carrying cables are multi-stranded 4 gauge "battery cables". The cheapest source on Amazon is 4 gauge jumper cables that you snip the ends off. $20 for 20 feet but you only need a few feet here. Shop around.
The load will need two 9 inch pieces of 3/4 copper pipe, a spool of .032 stainless safety wire and 24 small #6 stainless screws, the best I've found so far are the small screws that are used for aluminum gutters. You'll also need four 3 inch #10 stainless screws, they have to be stainless because it doesnt conduct heat like straight steel or especially copper.
This tester does not have any over discharge alarms or protection, you'll need to watch it like a hawk when you get near the end of the discharge. At 2.5V the cell is dead, remove it immediately or you risk damage to the cell. A better target is 2.7 or 2.8 volts.
Upgrades will be posted later for a low voltage alarm and to automate this with an automatic low voltage disconnect from the load. If there is enough interest I'll add onto that with how to automatically switch over from discharging to charging and toggle cycle counter using time delay relays. Somewhat complicated to do though.
We'll get into building the load and tester assembly in the next post.
Overall cost is about $130 US which is dirt cheap compared to buying a 60A tester.
In this example diagram the cell is a 120AH LFP cell, the charging power supply is a 60A Meanwell HRP-300-3.3, and a 60A load. There is a reason for these values:
Cell amp-hour capacity is usually measured at .5C with C being the capacity in amps. .5C means we'll be pulling .5 of its capacity which in this case is 60 amps and the battery will be exhausted in 2 hours. .25C would be 30 amps, etc. You can use a smaller charger but it makes the test that much longer. As it is a full test cycle takes 4 hours or so.
The power supply is a constant current limited power supply which means it'll reduce its voltage to avoid over-amping itself. If you do not use a current limited power supply it'll just shut itself off, start turning on and off until the overload is cleared, or just permanently die. This Meanwell will actually put out about 70 amps initially but because the cell voltage rises while charging and will drop below 60A fairly quickly. In addition the PS has a range of 2.8-3.8V and will need to be adjusted to 3.65V out of circuit (no load or battery) using the little phillips screw head near the power light. There are lots of meanwell clones out there if you hunt around. Whatever you use it has to be current limited.
The HRP line goes from HRP-75 all the way up to HRP-1000 (75 to 1000 watts) depending on how high you want your charging amps to be and how much you want to spend.
The best price I've found new is on Ebay for $75 including shipping, you can often find them used for a lot less.
The 12V source can be another power supply, or a 12V battery on a trickle charger. The meter only draws 2 watts so you dont need much. The Amp-Hour meter needs it's own separate power supply because LFP cells do not provide enough voltage for it to operate.
This is all assembled on a 24"x36" sheet of 1/2" plywood.
The meter is a PZEM-015 with 300A shunt can be found on Amazon for about $20 US. It comes with some wire but they tend to be a bit short, you'll probably need some extra stranded 18 gauge wire.
The amp-hour meter also displays voltage, current amperage, time watt hours and resistance. The only thing we're concerned with is the voltage, amperage and amp-hours. I prefer to use the shunt vs the hall effect amp hour meters because they're more consistant and accurate.
The 150A Fuse/Switches can again be found on Amazon for $16 but less expensive fuse/switches can be substituted, I just liked the convenience of the connections. Keep in mind these types of fuses' amperage are often badly overrated, in my tester I'm using the 300A versions.
All the main current carrying cables are multi-stranded 4 gauge "battery cables". The cheapest source on Amazon is 4 gauge jumper cables that you snip the ends off. $20 for 20 feet but you only need a few feet here. Shop around.
The load will need two 9 inch pieces of 3/4 copper pipe, a spool of .032 stainless safety wire and 24 small #6 stainless screws, the best I've found so far are the small screws that are used for aluminum gutters. You'll also need four 3 inch #10 stainless screws, they have to be stainless because it doesnt conduct heat like straight steel or especially copper.
This tester does not have any over discharge alarms or protection, you'll need to watch it like a hawk when you get near the end of the discharge. At 2.5V the cell is dead, remove it immediately or you risk damage to the cell. A better target is 2.7 or 2.8 volts.
Upgrades will be posted later for a low voltage alarm and to automate this with an automatic low voltage disconnect from the load. If there is enough interest I'll add onto that with how to automatically switch over from discharging to charging and toggle cycle counter using time delay relays. Somewhat complicated to do though.
We'll get into building the load and tester assembly in the next post.
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