Measuring Charge Efficiency

[redpoint5]

Originally Posted by 2000mc

Unfortunately this looks like it would at best save about as much as having a taillight burned out.
What I'm thinking is that this would be the same as a float charge. Battery tender has some charts putting float charge at 50-100mA

Agreed. Charging and discharging efficiency is a miniscule drop in the overall efficiency of petrol vehicles. Charge efficiency only needs to be considered in systems where they are heavily used, such as EVs, alternator deletes, and hybrid traction batteries.

Regardless, I'm curious, and I don't see much information available from internet searches. My ultimate goal is to do an alternator delete while preserving as much convenience and longevity of the battery as possible.

[sendler]

Why not just test on the bench with a charger and a consumer?

[redpoint5]

Originally Posted by Old Tele man

TRANSLATION: "consumer" = dummy "load"

I thought you were calling me a dummy load.

I brought my LiFePO4 and Supercap to work along with some resistors and meters and should be able to complete some tests. I'll start with 10% depth of discharge and then go down from there to 50% DoD and chart efficiency.

[IamIan]

If helpful.
For A123 Flavor LiFePO4 20Ah cells.

Over a extended period of time (months) they have much much less self Discharge at any given temperature than more conventional PbA.
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Available discharge capacity is reduced at lower temperatures. Similar effect in PbA (but different magnitude).
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The complete cycle (Charge + Discharge) efficiency (Ah or Wh) reduces at higher rates (A or W).
Varying discharge rates Link

With no other changes (rate,temperature,etc) the cycle (Charge + Discharge) efficiency ( Ah or Wh ) reduces at high SoC/SoE.
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The electrical resistance reduces at Higher SoC/SoE , but Increases at lower temperatures... Less resistance = Higher efficiency ... Agrees with above efficiency vs SoC/SoE measure.
Link

Peukert Effects of reduced Ah/Wh are almost entirely in the direction of the increased rate ... meaning if your Discharge rate increases but your charge rate does not ... less Ah/ Wh will be seen on the discharge part of the cycle ... and the opposite is also true ... if your charge rate increases but your discharge rate does not ... less Ah/Wh will be seen on the charge part of the cycle ... this effect is almost entirely on the CC portion of charge or discharge ... if a CV stage to charge or discharge is also included very little of this effect will actually be seen ... although, the time of the CV portion will greatly be effected by the CC rate portion.

• I saw less than 1% deviation on the charge (Ah or Wh) from deviation on the discharge rate from 10A to 100A.
• The end of CC portion moved ~5% (SoC/SoE) away from the bottom for discharge varying from 10A to 100A.
• I saw less ~0.5% deviation on the discharge (Ah or Wh) from deviations on the charge rate from 10A to 75A.
• the end of CC portion moved ~5% (SoC/SoE) away from the top for charge varying from 10A to 75A.

Taking what we know above combined with the OEM power vs SoC graph.
Link
We can also see a significant preference in allowable power pulses in the discharge direction ... Not just raw Watts ... but at the top SoC the Regen hits the 3.8v listed @ ~900W that's ~236Amps ... but on the opposite end the bottom discharge hits the 1.6v listed @~800W that's ~500Amps... which is also interesting considering the deviation we saw above in the resistance changes at those different ends of the SoC band.

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I used the following:

• 3 PowerLab8's connected together (up to 120A rates charge or discharge combined).
• VA18B multi-Meter
• Instek GPD-2303S Power Supply
• 36 NiMH 6 cell Sticks from HEV Civic for high power for the PL8s.

~32MB compressed raw data from years of testing if desired

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Forgot to add ... these cells prefer to be kept bellow ~120F , for best life.
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