Low mileage charging

[wes4dbt]

I just got a new set of 40ah Gel Mk's for my daily use chair and want to make sure I remember the numbers correctly. I believe I was told that even if you only go less than a half mile, say .4mile, the batteries should get a 8 hr absorption charge.

My daily use is pretty low. Average day probably 1 to 2 miles. A couple of days a week I don't leave the house, my use is probably less than a 1mile. Normally I like to get out of my chair in the afternoon for @ 2 1/5hrs and I always put my chair on the charger. Then again back on the charger when I go to bed at night for at least 8hrs.

My settings,

Absorption - 28.2
Float - 27.3
Storage - 26.7

Just want to make sure that isn't to much absorption charging.

[biscuit]

28.2V is the correct voltage for up to 8 hours. The current will reduce steadily over the 8 hours. When it reaches the point where the battery is drawing 40mA then is the time to switch to the lower voltage. It might be before 8 hours is up. My usage is about the same as yours and my batteries are 33Ah gel and usually take about 6 hours to be charged to where they draw 33mA.

[Burgerman]

The things that kill off lead while charging are not fully removing the natural lead sulfate that occurs when you discharge as this is a part of the reaction.
It is returned back to the electrolyte as you charge. If it is ony 99% returned, that leaves just 1% of the sulfates remaining on the plates that turn to large white crystals (over time) that cant then ever be recovered. As in this state they do not conduct electricity. That means that you would lose about 1% of the capacity every cycle.

KINDEST way to charge, is at say float CHARGING voltage. At 13.8V. This typically isnt very time critical. And so a full charge is achieved but it takes typically 20 to 30 hours. We dont have 20 to 30 hours... And its the LAST part that takes the time. That last couple of percent. You will see this stated as "float charge" on most spec sheets. This can be done for 2 days, maybe absolute max 4 days. Because its still a positive charge voltage. Meaning that once the battery is full the charge voltage is still ABOVE the battery normal full voltage and so some current continues to flow. Over time this causes plate corrosion etc.

So not FULLY charging causes sulfation, and continuing to charge after its full causes internal corrosion and possible gel shrinkage over time.

So if we have a lot of time, 2 to 4 days, 13.8V is great. It then typically is great for solar, or standby generators etc as we are not trying to do it fast. And are alsways topping off. Because it gets used during the day and we cant control solar hours.

If STORING a battery is required, then we need to keep it 100% full, with the lowest current flowing through it as possible. To cobat its internal losses and self discharge of a few% per month. This is typically 13.3V at room temprature. A few mA only, holding the battery about 1/10th of a volt above its naturally fully 100% charged voltage. This means no sulfation, and almost zero internal corrosion from having a very tiny current flowing indefinitely. But this would take at least a week or longer to charge a battery. And it can fail to balance cells. Since the battery full voltage and the charge voltage are almost the same. Its about 1 to 2 tenths of a volt different. So very little energy flows once the battery becomes almost full. The last part of the essential 100% charge can take a week or more. It allows idefinite storage at the slowest possible degradation.

If charging in cyclic daily use is required as we do, overnight as in our usage, then we have a problem.
We only have 8 hours typically. So we raise the charge voltage. The CV voltage. This means that instead of the battery taking a really low current once nerly full, it continues to charge much faster. Because the battery voltage and the charge voltage are about 1 full volt different on gel. And 1.5V different on AGMs.
So now the final part of the charge where the current tails off is much more agressive. Now we are forcing more current in. This results in a typical CC stage to abount 80%, and absorption stage that in 4 to 5 hours has returned 95 to 98%. So about 7 oor 8 hours and the battery is nearly full.
But now its very hard to know when to stop.
We do not want to leave the battery undercharged. That would happen if we stopped at say 5 hours CV/Absorption. Although a high float stage after this WOULD eventually get it to 100% if we wait long enough. Thats what most mobility chargers do. Albeit at wrong voltages.
And if we take a typical gel, like the MK in heavy cyclic use, say 50% discharge, then we could say 8 hours CV. The problem is that if we only have a small discharge level, like say 5% or 10% then it may take less time than this. So now we dont know whan to stop.
Except MK tells us that we should stp when the current stops dropping by 0.1A over a 1 hour period, or if it begins to rise again, or if 12 hours at CV has elapsed.
But theres also another way. That takes a little testing and configuring. That is termination by watching tail or CV current falling away.
The reason that it takes a bit of testing is that different batteries and different depths of discharge and even previous charge and usage history makes a differenc as does temperature.
On say the Odyssey its clear. They say CV stage IN CYCLIC USE until 1000th of capacity (tail current) is reached at 14.4V to 14.7V. Or 8 hors whichever is sooner.
With MK gels I would do the same at 14.1V and same 8 hours limit and 1/500th C. (So terminate at 150mA on a 75Ah gel battery). Why? Because gel is very unpredictable and current ay not drop as expected. And so best to terminate a litle earlier and allow a float voltage to top it off later. IN CYCLIC USE.

I know you want a more simple answer. There isnt one.
I adjust mine acording to depth of discharge estimate, summer/winter and old or new. As well as battery brand.
One way to get a rough idea is to charge a half discharged battery. Set the CV voltage to cyclic 14.1V for gel, and watch the current drop over 8 hours, when it reaches 8 hours look at the mA current. Set that as termination, and add say 10% to the figure. IF it stops falling by 0.1A in any 1 hour period, look at this level of current, regardless of time, and set that as the termination current.
And keep your eye on it periodically to check its behaving.

[wes4dbt]

Thanks for the reply,

One way to get a rough idea is to charge a half discharged battery. Set the CV voltage to cyclic 14.1V for gel, and watch the current drop over 8 hours, when it reaches 8 hours look at the mA current. Set that as termination, and add say 10% to the figure. IF it stops falling by 0.1A in any 1 hour period, look at this level of current, regardless of time, and set that as the termination current.

When you say, "Set that as termination", are you saying go from Absorption to float voltage at that time? I have a Victron IP65 charger am not able to set a "termination current". It does have an "Adaptive mode" that will decide when to go to float and storage but it seems to go to quickly.

[Burgerman]

Yes and thats a problem.

Of course its only a problem if you care. Anything can actually charge lead bricks. As shown by all the nasty incorrect mobiliy chargers.
It all just depends how much you care.

Heres MKs take.
Tell me if you dont understand anything here.

www.wheelchairdriver.com/MK1.pdf

[wes4dbt]

Yes and thats a problem.

Of course its only a problem if you care. Anything can actually charge lead bricks. As shown by all the nasty incorrect mobiliy chargers.
It all just depends how much you care.

Heres MKs take.
Tell me if you dont understand anything here.

http://www.wheelchairdriver.com/MK1.pdf

Thanks and I will read that in a little while. I got to do a little shopping now.

I'll almost check my current after my 2 1/2hr charge and overnight charge. Perhaps that will help to decide on how long the set the absorption.

[Burgerman]

I know a long read.
But 1 more paragraph or so here as it will help clarify why charging is hard. And no fixed way.

a) 13.3V not time sensitive. Maintain only. 10 years is fine but takes weeks to fully charge.

b) 13.8V charge is VERY time sensitive as anything from 2 to 4 or 5 days is OK. 2 days should give full charge. But its a bit more time critical than above.

c) 14.1V gel, 14.6V AGM. Fairly fast charging, so overnight is nearly possible. Needs about 2 hours at CC and 8 at CV. But now overcharge can become problematic, even thermal runaway is a danger if charged too long. And so it becomes harder to know if its full, or overcharged.

d) another method we didnt discuss. Same 14.1V and when it arrives at a typical 1A tail current. FIX current at 1A and charge for 2 hours. This is done with airport floor cleaning machines etc. Because an 8 hour obernight shift on charge is not adequate to wait while current falls really low. So now we let current fall to say 1A and then maintain 1A for 2 hours ignoring the voltage. There are other variations of the theme. We typically stop if voltage exceeds 15.6V or similar.
This is called the rapid finishing stage. It shortens recharge time to around 8 hours total. In deep cycle use. But now its super easy to overcharge. Or undercharge. And we dont really know how much extra to pump in unless we measure Ah out as we use the battery. And so now charge time is super critical.

Its all about time.
These methods all rsult in a full battry. But some take several days aor a week. Some take 8 hours. Or less if not fully discharged.

So you can see that whan I charge I think of how discharged it is, how long I have, how warm or old the battery is, and decide on a voltage, time, end current, if float or rapid finishing stage is best. And I dont alsways do the same thing.

E.G watching motoGP on a sunday morning? Then at midnight after the pub I maybe have 12 hours on my bed. So 14.1V (14.0 in summer) and 6 hours CV or whenever it seems o stop falling noticibly in an hour, and then a 13.8V float to end until I get up.

If I am up at 8, then I may do the rapd finishing stage.

Sat in the next room for months? 13.8v 24h, then 13.3V indefinitely.

Etc.