# How many watts to charge a li-ion battery

Posts: 6Registered Users ✭✭
If my battery is rated at 2200 mAh, how do I estimate how many watts I expect to use recharging it? I also have a dc adapter.
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• Posts: 298Registered Users ✭✭✭
edited September 2017 #2
Does your battery have a voltage rating?
Do you know what the formula is for Watts?

• Posts: 6Registered Users ✭✭
Oops. Its 12V. I don't know how to calculate watts from mAh's
• Posts: 7,916Solar Expert ✭✭✭✭
Li-Ion is pretty efficient.  If you use 342watts, 350watts would recharge it pretty close to full again.
But to figure out a solar charger for it, you better stick to about 90% of battery capacity, to use for your solar harvest guesstimate.

2.2Ah x 12V = 26 watt hours in that battery.  So a 50w panel, with average placement (25w output) could charge it in an hour.
Unless its too late tonight and I'm not doing the math right
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Posts: 2,846Registered Users ✭✭✭✭✭
The battery is pretty efficient, but the DC adapter (aka wall wart) may not be. You may be able to make a guess on this by looking at in input and output values printed on it.

For example (made up values), say it's 0.5a @ 120vAC input, and 2a @ 12vDC output. That would be 0.5x120=60w in, which is the rate of power use. If it takes two hours to charge, total energy used is 120w.

2x12=24w is going to the battery. The difference, 60-24w = 36w is going to heat the wall wart. Also, some wall warts can use power just being plugged in. I put mine on a power bar which gets turned off when not in use.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter