Thread: Battery Charger Float Mode

I have a Ctek MXS 5.0 and every so often charge the D4 batteries overnight.
I have a Traxide system - SC80 isolator between crank & Optima yellowtop aux 55Ah.

Had the doors open and lights on a lot over the weekend doing stuff and got a dash warning which I think happens at 12.2V
So charged overnight last 4 nights in a row and used the iiD tool to read battery SoC as recorded by the vehicle BMS each morning.

All SoC measurements with D4 running and remaining constant over a 40 minute drive:

26/3 - 60% (due to weekend activities)
27/3 - 74% (after o'night charge)
28/3 - 78% (after o'night charge)
29/3 - 83% (after o'night charge)
30/3 - 86% (after o'night charge)

Now each of the charging nights was around 9-10 hours continuous and at least 8 hours on FLOAT.

I always thought FLOAT meant the battery was fully charged before the charger went to that mode.
But these SoC figures suggest otherwise.

I realise that the SoC could be incorrect due to having an aux battery connected into the system.
Could that be why the SoC is not higher after 8 hours of float?

I don't think anything is wrong or of concern here, I am simply interested in what is going on with the BMS SoC reading.
Is it real or skewed by the aux battery which is like a 50% increase in battery capacity or by the battery charging?
And if so why because surely float mode would apply to the entire connected load of both batteries.

Regards,
Scott

Hi Scott, and your results are exactly what you should be seeing.


But first, prior to starting your charging test, did you do a lot of short drives, and I mean over say the last few months?


If so, then you have been slowly reducing the USABLE battery capacity.


Then by charging every night, the FLOAT portion of the charge cycle has been slowly recovering lost battery capacity.


If you use a digital multi meter to measure the voltage in the morning, you will probably get a fully charged indication.


This is partially correct. The voltage reading relates to the State of Charge ( SoC ) for amount of USABLE battery capacity available, not for the original total capacity.


Whereas, your BMS is relating to the percentage of battery capacity available, based on the original total battery capacity.


Both results are accurate and can be used together to keep an eye on you battery capacity over the lifespan of the battery.


A suggestion for you, keep carrying out your nightly chargers and see how close to 100% you can get.


BTW, your overnight cycles are actually much more effective than just leaving the charger on continually.


Also, there is no need to separate the batteries while you charge them. They will both eventually reach the maximum state of charge independent of one another even while connected together while charging.