Runtime and mah: lithium ion vs Ni-MH

[StephenD]

Hi all,

First time poster but long time beneficiary of your collective wisdom (thank you!), hoping for some help on something that's confusing me! To take two examples, the olight i3t with an eneloop NiMH 930mah will do 5 lumens for around 16 hours. The baton 3, with apparently a 550mah lithium ion, will do 12 lumens for 33 hours. My understanding was that, regardless of battery type, the higher mah battery should run for longer, especially since these are the same company, similar led's etc i.e. I presume similar efficiency in the actual flashlight design, but hugely different run times.

Can anyone explain to me what is going on here? The only thing I can think is that the lithium ion is somehow more efficient when producing more than '1' milliamp i.e. for the '930 / 550 mah' statistics, when producing 1millamp the nimh may last longer, but when producing higher e.g. 20milliamps or whatever is needed to run 5/12 lumens, the lithium ion will do this more efficiently and thus run for longer??

Thanks so much,

Stephen

 

[archimedes]

Hello and :welcome:

I have approved your post above, and moved it to our "Electronics / Batteries" forum.

Oh yeah, and ... voltage

 

[Lynx_Arc]

As arch said, voltage. The nimh is 1.2v nominal while the lithium ion is 3.6v nominal which translates into 3 times the power for the same mah. Another thing to consider it LEDs require around 3-3.5v and a nimh is too low to supply that in a single cell so what they have to do is incorporate a voltage boosting circuit the translates higher current at a lower voltage to higher voltage at a lower current and with that conversion is some power loss involved as the circuitry isn't 100% efficient. Lithium ion doesn't need a boost circuit normally for LED lights so there is a lot less to almost no loss to step down the voltage a few tenths of a volt. If you use 3 nimh in series you get about the same voltage as a lithium ion. One other thing to also consider is power density, of which lithium ion wins hands down it has more power (watts) per volume and weight than nimh does. This is why most devices have gone to this battery type and a lot of us have moved to lights using the technology especially high output lights.

 

[badtziscool]

Yup. As it has been pointed out, you have to consider the nominal voltages of the batteries as well as its capacity. NiMH is 1.2v nominal and Li-Ion is 3.7 nominal. So you take the voltage and multiply by mah, and you will get power capacity of the battery and that is a much better indication of the "capacity" of the battery.

 

[idleprocess]

Watt-hours are the trick here, not the amp-hours on the cell faceplates - something that Lynx_Arc and badtziscool have alluded to.

• NIMH : 0.9Ah x 1.2V = 1.08 Wh
• Li-ion : 0.55Ah x 3.6V = 1.98 Wh

About double the Wheaties™.

Boost circuits also hit battery life since they have to draw more current to boost voltage while delivering similar amperage to a buck circuit to the same LED; this hurts yield on a cell since higher current draw means worse Ah due to internal resistance and heat loss. The inherent efficiency of a boost circuits seem to be less than that of buck circuits as well.