Question about battery operating time.

[arizau]

I have a small battery that is rated at 12.8V and 6.4 amp hours. Suppose it is connected to a 12V motor that is rated to draw 1.25 amps. 6.4 amp hours divided by 1.25 amps equals 5.12 hours and indicates that is how long it will operate at continuous discharge but info on the battery indicates it will shut down when the voltage reaches 10. That, to me, indicates that voltage drops while the battery is being discharged, or does it(?) since when I use this same battery on my dry washer, that motor (no specs available) does not slow down as an indication of low battery, it just suddenly stops running. I was told that was what would happen. Any idea how long it should actually run a motor rated at 1.25 amps?

 

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[ratled]

Battery math doesn't work like regular math.

First off, the type of battery it's will dictate how much of a full tank you can actually use. So if you are using a small deep cycle battery vs a small tractor starting battery with the same specs you will get two totally different answers. You can't run either down to zero or you wreck the batteries. A simple lead acid you might be able to run it to 80% (use 20% and run to a rating of 80%) and gel cell a little more. An AGM style might get you to use 50%. A lithium battery might allow you to use 90% of that full tank.

ratled

 

[Capt Nemo]

The 5 hour run-time should be correct. It will become shorter as the battery ages.

 

[WaProspecting]

I went out a few times with a solar and it worked. Wasn't that much weight even with the batteries... They are pretty cheap these days and durable.

How old is the battery and does the motor randomly speed up at times or slow down while using it? You can't really tell unless you actually run it after fully charge. You also would have to maximize the load and friction because out in the field it's different.

 

[arizau]

The battery is a LiFePo4/Lithium Iron Phosphate type and only weighs about 2lbs. Supposedly good for up to about 2000 recharges. Came with the drywasher but I'm thinking about also using it on another piece of equipment.

 

[Jim in Idaho]

Li-ion generally stays at the nominal voltage of 3.7v/cell, until they get close to 3.0v/cell. For the best life from the pack you shouldn't go below 3.0v/cell, but many manufacturers say you can run theirs down to 2.5. Your's is a 3-cell pack. Can be charged to 4.2/cell, or slightly higher, thus the 12.8volts, but will go down to the 11.1 nominal fairly quickly...then stay at that level to about 9.0 volts, and go on down from there.
The amphour rating is probably a little exaggerated, and will also decline over the life of the pack.
Jim

 

[Reed Lukens]

The battery is a LiFePo4/Lithium Iron Phosphate type and only weighs about 2lbs. Supposedly good for up to about 2000 recharges. Came with the drywasher but I'm thinking about also using it on another piece of equipment.

Just add up the amp hours on the equipment that you will be using then subtract it from the 6.4 amp hours to see what's left. If you're planning on using it on a small bilge pump, they draw a lot more then what is stated. Usually 7+ah. I run around 9ah of equipment on my 32ah LiFePo4 and can run it all day with the 100watt Renogy Solar Suitcase hooked up also because it's bringing in 7+ amps itself on a sunny day. A 6.4ah battery is pretty small but when backed by solar, anything is possible.

 

[standard8]

Many lithium batteries with cobalt as one of the ingredients do charge to 4.2 volts per cell and bottom out around 3.5 volts. But lifep04 has no cobalt and has a full charge voltage of 3.65 volts and low end can be as low as 2.5 volts. Plus high cycle life is possible.

 

[arizau]

Li-ion generally stays at the nominal voltage of 3.7v/cell, until they get close to 3.0v/cell. For the best life from the pack you shouldn't go below 3.0v/cell, but many manufacturers say you can run theirs down to 2.5. Your's is a 3-cell pack. Can be charged to 4.2/cell, or slightly higher, thus the 12.8volts, but will go down to the 11.1 nominal fairly quickly...then stay at that level to about 9.0 volts, and go on down from there.
The amphour rating is probably a little exaggerated, and will also decline over the life of the pack.
Jim

Not sure we are on the same page since the chemistry is a little different and it is one manufactured case not like the shrink wrapped battery packs I have seen online and maybe you are referring to. Here is the one I have. https://www.amazon.com/Energy-K2B12V7EB-Lithium-Phosphate-Battery/dp/B0056BXE6A. Pretty pricey but super light weight. Looks like Reed uses the same brand but with higher capacity.

Needless to say, I am electrically ignorant thus the reason for my original post but I have gotten a little more knowledgeable with each response so thank you all.

 

[Jim in Idaho]

You're right! I didn't realize the extent of the differences. here's what Wiki had to say about that type of battery.
"he LiFePO
4 battery uses a lithium-ion-derived chemistry and shares many advantages and disadvantages with other lithium-ion battery chemistries. However, there are significant differences.

LFP chemistry offers a longer cycle life than other lithium-ion approaches.[11]

Like nickel-based rechargeable batteries (and unlike other lithium ion batteries),[12] LiFePO
4 batteries have a very constant discharge voltage. Voltage stays close to 3.2 V during discharge until the cell is exhausted. This allows the cell to deliver virtually full power until it is discharged. And it can greatly simplify or even eliminate the need for voltage regulation circuitry.

Because of the nominal 3.2 V output, four cells can be placed in series for a nominal voltage of 12.8 V. This comes close to the nominal voltage of six-cell lead-acid batteries. And, along with the good safety characteristics of LFP batteries, this makes LFP a good potential replacement for lead-acid batteries in many applications such as automotive and solar applications, provided the charging systems are adapted not to damage the LFP cells through excessive charging voltages (beyond 3.6 volts DC per cell while under charge), temperature-based voltage compensation, equalisation attempts or continuous trickle charging. The LFP cells must be at least balanced initially before the pack is assembled and a protection system also needs to be implemented to ensure no cell can be discharged below a voltage of 2.5 V or severe damage will occur in most instances.
Jim

 

[arizau]

You're right! I didn't realize the extent of the differences. here's what Wiki had to say about that type of battery.
"he LiFePO
4 battery uses a lithium-ion-derived chemistry and shares many advantages and disadvantages with other lithium-ion battery chemistries. However, there are significant differences.

LFP chemistry offers a longer cycle life than other lithium-ion approaches.[11]

Like nickel-based rechargeable batteries (and unlike other lithium ion batteries),[12] LiFePO
4 batteries have a very constant discharge voltage. Voltage stays close to 3.2 V during discharge until the cell is exhausted. This allows the cell to deliver virtually full power until it is discharged. And it can greatly simplify or even eliminate the need for voltage regulation circuitry.

Because of the nominal 3.2 V output, four cells can be placed in series for a nominal voltage of 12.8 V. This comes close to the nominal voltage of six-cell lead-acid batteries. And, along with the good safety characteristics of LFP batteries, this makes LFP a good potential replacement for lead-acid batteries in many applications such as automotive and solar applications, provided the charging systems are adapted not to damage the LFP cells through excessive charging voltages (beyond 3.6 volts DC per cell while under charge), temperature-based voltage compensation, equalisation attempts or continuous trickle charging. The LFP cells must be at least balanced initially before the pack is assembled and a protection system also needs to be implemented to ensure no cell can be discharged below a voltage of 2.5 V or severe damage will occur in most instances.
Jim

By virtue of the second to last paragraph it seems that constant full power is available until battery charge is exhausted so, in my case, it should operate up to about 5 hours as long as the battery is in good condition and fully charged. Should meet my potential needs.

Thanks

 

[KevinInColorado]

The Whippet comes with these batteries because they are the most power per pound! I love mine.