Charger efficiency?

[itchynackers]

In trying to figure the true electricity cost for charging from my home outlets, it dawned on me that I need to somehow approximate my charger efficiency. I do have a good data curve for how much my batteries put out as opposed to the watt-hours put in, however there seems to be a lot of heat lost through the charger. Does anyone have any rough approximations for cheap chinese 36V 2A non-fanned chargers?

Thanks,

Adam

 

[Tiberius]

Difficult to say. I would suggest starting with 75% in your calculations. Then if it turns out to be a critical factor, go back and look for figures for specific equipment.

Nick

 

[Lock]

$22 plus $6 shipping:
http://www.weemscreeksolutions.com/KillAWatt.htm

I believe most charging efficiency isn't linear too... The last few percents to get to 100% Ah capacity are usually less efficient, so ya can pull the plug at close to 100% and increase overall eff per charge

lLok

 

[jag]

First, 75% efficiency in a switched power supply as Nick suggested is a good first "null" hypothesis, but 80-90+% is achievable. Unfortunately these figures are seldom published for "consumer" PSU's and chargers. Recently I had to measure and select among possible PSU choices for a mobile manipulator (robot). Efficiency of tested units varied with input voltage and load. Though the units we tested were speced for 110-220V operation, some exceeded this by a wide margin and would work down to 40-50V. These typically had their best effiency of about 90% at these low input voltages. Others did not work much below rated voltage. These measurements were made by measuring watts in and out.

Another way to measure efficiency is to measure the dissipated heat. This can be done fairly accurately for a small object like a power brick style PSU/charger.

Needed:
A thermometer,
a heat absorbing material with known thermal heat capacity,
a vaccuum bottle/thermos big enough to fit all. (like the 1-2 litre big mouth food jars)

Procedure:
1. Put charger/PSU, bulk material and thermometer in vaccuum bottle. Seal top as well as possible.
2. Close the bottle. Since the cap won;t screw on with the wires hanging out, stuffing the top with cotton (or for the fainthearted some non-flammable insulation) is a good idea.
3. record start temp T1 and start time t1.
4. Run charger/psu and watch temp increase.
5. Turn off at some reasonable point, say 50C, record end time t2.
6. IMPORTANT: Wait for equilibrium. At the turn off point the thermometer is likely to still increase. Wait until it stops increasing, then read final temperature T2,
7. Calculate the total heat capacity C of the stuff in the vacuum bottle.
8. PSU loss (Watts) Ploss = (T2-T1)*C/(t2-t1)

Finally PSU/Charger efficiency is Pout/(Pout + Ploss)

 

[jag]

Some notes:
First credit to this procedure goes to my high school physics teacher Stig Sewik. We used it in class to measure the caloric value of various fuels.

Point 7: While I've used air, a material with higher specific heat coefficient is desirable. Water would be good if the charger is water proof. But water can conduct electricity (some fellow students used this in a prank to electrify the urinoar in the mens room). A block of metal can also work well for bulk heat absorbtion.

Point 6: If a solid material is used for heat absorbtion, taping it to the PSU reduces the wait time until equilibrium.