Drifter battery capacity calibration.

iana · Guru

Drifter battery capacity calibration.

Tony Bev · Guru

Another good and interesting post, Ian

I am not a techie, but like many others, I would appreciate a techies point of view, on this question

My point of view is, after reading the instructions, I would never deliberately run my batteries down to 10.5 volts

Peter_n_Margaret · Guru

There is a very commonly believed urban myth that running AGMs down to 10.5V damages them (10.5V is the accepted zero state of charge point).
I suggest that this is not so.
Doing so on a regular basis will have a relatively minor effect on total life. Life being measured by the total power delivered over the life of the battery, not "cycles" which is a meaningless measure..
Cheers,
Peter

T1 Terry · Guru

It is up to you to decide at what point you consider the battery to be fully discharged. The battery manufacturer uses a voltage per cell and somehow this gets related to a total voltage at the terminals, 1.75v per cell x 6 cells = 10.5v. That only works if all the cells are at 1.75v when the 10.5v point is reached. What if one cell isn't doing as well as the others? The same 10.5v could also be 5 cells at 1.85v and one cell at 1.25v and a damaged cell.
A safer minimum voltage is around the 12v mark or 2.0v per cell, so stopping at 11.8v would give you a point where you considered the battery to be as discharged as you ever wanted to go and this should become the 100% used point or 0% capacity remaining.

You might have to put in half of the advertised capacity as the battery size so the Drifter doesn't decide your batteries are no good, no idea if these units are that smart but you never know.
The charging to 100% then leaving the charger on for another 24hrs backs up my posts regarding recharging a working battery to 100% via solar, you might get up into the high 90% mark but you won't get it back to 100% on solar alone.
So, discharging to 11.8v and then recharging for 3 cycles will not harm the battery and hopefully give you a % scale of state of charge that reaches 0% when you have used all you want to use out of the battery each time. In reality this would probably be around the 50% state of charge for a brand new battery and help to get the max number of cycles before the battery degrades to the point there is only 50% of the capacity left. This will be indicated by the voltage dropping to below 12v not long after the sun has gone down and it is a very slippery slope from there to virtually no capacity left in the battery.

Not sure if that helps or not, most systems I get involved with still have the drifter connected by the Victron BMV is the one that does the accurate state of charge because it is actually counting the Ah out against the Ah in and adds the percentage lost in each direction. They don't need to torture the battery to learn, you can simply program in the relevant information. The drifter can not measure any high loads like an inverter or hydraulic levelling legs or a bed lifter etc, so they are rather limited in their functionality.

T1 Terry

-- Edited by T1 Terry on Saturday 2nd of March 2019 03:39:44 PM

-- Edited by T1 Terry on Saturday 2nd of March 2019 03:41:14 PM

Izabarack · Guru

iana wrote:
..... to go through the calibration process and be assured no harm will be done to the batteries. And to be able to gauge battery capacity by amp hours left in the battery via the shunt and the bar graph reading.

That assurance is dodgy in my view. Anything lower than 11.6 is getting very risky for a Lead Acid battery.

The bar graph reading will not be that accurate as it will be based on a previous calibration done on the previous battery set.

This procedure of a repeated full charge followed by a discharge to a voltage setting that is considered to be flat, is commonly used in other places, especially for Li Ion batteries The amount of energy required to reach full charge is recorded then how much energy is given up getting to the user determined flat point, is also recorded. This cycling may be done three or four times to check. by the repeated and recorded charge and discharge, the usable capacity of a specific battery can be established. The difference to the Drifter method is that full charge voltage is first established by charging until charge current is less than 200mA followed by a rest for at least 8 hours. The terminal voltage of the rested battery can be relied on as the voltage of that specific battery when fully charged. The second difference to the Drifter procedure is that the user determines just how low they want to take the battery before they want to recharge, as a way of ensuring long life. The Drifter procedure assumes too much and, in my view, takes the battery far to low.

You can follow the Drifter instructions if you choose. However, I suggest you choose a low voltage point of around 11.6 for your calibration. There may be more left in the battery past that point but if you calibrate your scale for between full charge as 100% and low point of 11.6 as the lowest you want to go, Zero%, the bar graph will be pretty accurate for your use.

Iza

Izabarack · Guru

Just another point for anyone interested. A device to establish battery capacity between the voltage you have decided is fully charged and the lowest voltage you want to take a given battery, can be had of as little as AUD$27 on line. I got one at the same time I bought some LiFePo4 cells to make up into a battery for the campervan. I want to charge the cells to 90% and discharge to 13% to give me a small safety buffer at each end. I currently get 43 Ah out of nominal 50 Ah cells using the method of measuring energy in and energy out to reach each end of my charge/discharge profile.

Iza

iana · Guru

Thankyou for that information guys, as we are camping at the moment using solar only, on our fourth week now, I can't carry out the calibration process at this stage, however one point brought up that I had not thought of was the use of an inverter, which I am assuming we will be purchasing in the near future.
It is the other half that is watching the bar graph, while I watch the voltage. I am not so concerned about the low side of the battery capacity as I have another battery we can plug in if things get down a bit in the wee hours of the morning. Its the battery recharge I am concerned about, and I will begin another thread on that.