Building an SCC, need help with charge profiles/algorithms

[Bernard in Ireland]

Hi all,

I'm Looking for some advice on charging regimes for Lead acid batteries.
I feel I’m going around in circles, possibly overthinking the problem. I’ve a strong background in electronics, but am new to Lead acid charging.
If someone could sanity check what I’m saying here, and maybe point me in some direction?

The product:

• 20W solar panel
• 20Ah Lead acid battery 12V.
• A Motor that get switched on for aprox. 2 minutes per day (taking aprox 5A)
• Custom designed Timer PCB charging circuit on it.

There will probably be a few hundred of the system made.

I have full control over Hardware (electronic design) and software.
The application isn’t particularly price sensitive… ie there is scope to double the battery capacity or PV panel capacity or whatever else helps make for a durable design.

The current consumption of the electronics is less than 20mA
The current consumption of the motors is 2 minutes @ 5A (worst case)

All in all, these two added together is small in the scale of the battery capacity (less than 3.5% of the battery capacity)

So, If the PV panel could ‘keep up’ on dull/rainy day, the battery would never discharge by more than 3.5%.
I’m trying to understand a ‘decent’ charging regime to use in a low (but not zero) drain situation like this.

Sanity check 1:

1. Constant current charging to get the bulk charge done, is probably not appropriate for this level of discharge. .
2. Topping charge (2.30 to 2.45 per cell) sounds good to me, but I’m told it’s not good to leave a battery at this voltage for an extended period. Sounds easy to achieve with a CV charger.
3. Float charge ballpark 2.27V also sounds sensible and is easy to achieve with a CV charger

(figures taken from https://batteryuniversity.com/article/bu-403-charging-lead-acid )

I guess My question is , in a situation like this, would a simple CV charger, with a well-chosen charge voltage, perhaps with a bit of temperature compensation perform quite well?
ie give ‘sensible battery’ life. 4-5 years would be nice.

Sanity check 2:
My understanding of a CV charger is simply one gives more current to the battery as the voltage drops below the set point (like a fixed voltage and a series resistor would)
Eg
2.3V 0 ma
2.2V 200ma
2.1V 400ma
2.0V 600ma
(in each case above currents obviously further limited by the solar power available at any particular time)

Any pointers would be much appreciated.

Bernard

 

[sunshine_eggo]

Depending on battery specs, simple float at 2.27-2.30V/cell.

If battery voltage dips below 2.08V/cell, run an absorption cycle to 0.02C tail current.

Run a single absorption cycle to 0.02C tail current every 30 days.

 

[Warpspeed]

Everyone will suggest something different.
Every modern car just uses an alternator set up to produce around 14.1v to 14.3v constant voltage charging and it works just fine.
No equalizing, no float, no frills.

You need to run a flooded vented battery though....
Any kind of sealed gell battery will vent and dry out, and be stuffed in a year or two, regardless.

 

[sunshine_eggo]

Everyone will suggest something different.

True. Some will make more sense than others.

Every modern car just uses an alternator set up to produce around 14.1v to 14.3v constant voltage charging and it works just fine.
No equalizing, no float, no frills.

Not true.

Have you checked "modern" charging systems in the last 20 years? Extremely common for vehicles to have an adaptive charging algorithm. Honda's been doing it for 22 years since the Insight. They routinely let the battery drift to 12.7-12.8V resting unless one turns their headlights on. Then it floats at around 13.5V.

Prius has been doing 14.2V absorp and 13.5V float since 1997.

I've driven many rentals where I used a voltmeter/USB charger in the cig lighter port, and I watched them slowly cycle up and down between 12.X and 14V.

This is all to eke out another 0.1-0.2mpg to help fleet mileage requirements.

The OP is essentially describing a standby power system. There's a model for that. Float AGM or GEL at 13.8V/full charge. Most batteries designed for this application last 10 years. Since its solar powered, and it will be lightly cycled, criteria for bad weather absorption charge and a periodic absorption charge are prudent.

 

[Warpspeed]

Its not really possible to continuously float a solar battery when there may only be a couple of hours of usable solar in one day, followed by a couple of days of zero solar.
You need to be able to bulk charge when the power is available.

Bulk charging a sealed battery that has very little water in the gel will cause some gassing and gassing will cause the battery to dry out very quickly.

 

[sunshine_eggo]

AND this is why you don't cross post...

Charging lead acid when there is no 'charge event'

Hi all, This is my first post here I'm Looking for some advice or pointers on charging regimes for Lead acid. I’m sure what I'm doing has been done before!! The product: 20W solar panel 20Ah Lead acid battery 12V. A Motor that get switched on for aprox. 2 minutes per day (taking aprox 5A)...

diysolarforum.com

 

[32 volt boater]

Actually what was messing with me was I turned on a bunch of stuff dragged down the volts and there should have been a charge event.

FM80, got to go through it again.

 

[Bernard in Ireland]

Many thanks. for tall the answers. And apologies for the delay.

The OP is essentially describing a standby power system. There's a model for that.

I guess this is what I was suspecting. House alarms etc etc

Depending on battery specs, simple float at 2.27-2.30V/cell.

If battery voltage dips below 2.08V/cell, run an absorption cycle to 0.02C tail current.

Run a single absorption cycle to 0.02C tail current every 30 days.

Understood, I'm going to work on each bit of this.

I suspect that 'Depending on the battery spec' bit is the devil in the detail.
The lead acid topic is broader than I imagined.

You need to run a flooded vented battery though....
Any kind of sealed gell battery will vent and dry out

This is more of it... I haven't done my homework on this side of it at all!

Ill start with a battery that is rated for 'standby' applications
The Yuasa NP Series advertised for "standby applications' seems to be sensibly priced and 'commonly available'

re. cross posting. In the first post MisterSandals response proved that the original post could have been written better, and he kindly pointed out that the title could be better. I had to agree. I will do what I can to indicate that the post is closed. I'm not a regular poster (indeed to any forum)

I've More reading to do.
Ill try and summaries the findings (at least in broad terms) in a few days for anyone coming after me that is designing a standby system.

This has been very useful.
Bernard

 

[Vigo]

Even though solar conditions fluctuate greatly, i think you are not going to have to worry about that much because the draw on the battery seems low enough that you could go a few days without charging before running into problems. Lead acids don't like to sit at partial state of charge for long periods, but from what I have seen and read long periods is generally more than a few days.

I recently watched several hours of various training webinars from Rolls Surrette and they mentioned making sure the batteries are fully charged once every 'week or two', although they were referring to flooded cells.