Tuesday, 25 February 2014

Ah VS Wh for Lithium Battery State of Charge

Following previous post, I've found out the in-line power meter that I bought from eBay does not count Ah and Wh properly. This one only counts properly when output current and power is constant. When the output drops to zero, the Ah and Wh also drops to zero. What the??

So, I managed to find this one from Altronics (Manson), and work as expected. Definitely not worth to buy el-cheapo in-line power meter, for any model (after googling around). Save yourself lots of trouble and get the manson one instead. So, here's my final circuit:

Trying to minimise my loss, I use the crap-o-meter to display Volt, Ampere, and Watt for solar panel output. The good one, I use for coulomb-counting for battery charging. I wasn't sure whether to use Ah or Wh to calculate the State of Charge of my Lithium battery pack. My original thought was using Wh, as surely energy is better indicator? However, I trust white paper more than some Joe-Blog opinion (like myself). ALL white paper that I found so far, mention 'Coulomb counting' (i.e. counting Ah) is one way to monitor your state of charge, so that what I'll do for my battery pack. One of example of the white paper can be found here.

From my own testing, charging using Ah seems more accurate than using the Wh. Here is the example:

I used 6.864Ah or 248.02Wh in one of my commuting day last week. I got this figure from my 'cycle analyst' installed in my electric bike.

When charging (with constant current 1.2A most of the time), I stopped the charging at 6.6Ah or 259.6Wh according to my Manson in-line power meter (using my solar charging circuit). At this time, the voltage of the battery pack was 41.78V. After almost 2-day rest of the battery, my battery pack voltage was 41.4V (using cycle analyst).

Hence, from the above, definitely use Ah, NOT Wh counting for reliable discharge and charging of your lithium battery pack.

As you may already know, most lithium battery pack specify both in Ah and Wh. For example, my battery pack says 36V, 11.6Ah, 416Wh. However, from my usage so far, there is a few Watt discrepancy between the actual Wh and Ah (by simply multiplying it with 36). So, again, using Ah, not Wh for battery State of Charge, is definitely more accurate.

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