Charging a 12V lead battery from a 18s system

Batteries, Chargers, and Battery Management Systems.
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HrKlev   100 W

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Charging a 12V lead battery from a 18s system

Post by HrKlev » Oct 19 2020 10:36am

Hi!
I've been searching a bit, but havent found quite what I am looking for.... Is there a device out there that doesnt cost a fortune, and can safely charge a 12v lead acid battery from lets say a 18s system? I want to keep 12v battery on an ATV conversion because of a 12v winch that demands a lot of current. Can I just use a normal 10A 60-96vdc to 12vdc power supply with some kind current limiting circuit? Sorry if it is a stupid question, Im no electric wizzard....
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ZeroEm   100 kW

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Re: Charging a 12V lead battery from a 18s system

Post by ZeroEm » Oct 19 2020 10:44am

I had a 72V - 12V 20a step down converter only lasted 6 months. After riding over a rough road it started poping and fizzing then died. May try another one but would rubber mount it.

Look like this
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HrKlev   100 W

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Re: Charging a 12V lead battery from a 18s system

Post by HrKlev » Oct 19 2020 11:43am

Thanks, but I dont think this would work by itself...? I actually have a 10A version here after another project. When the winch is used, the 12v battery will sag a lot, so if this is connected to the battery without some kind of current limiter it would go into overload...? And 20A is not enough to draw the winch alone.
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ZeroEm   100 kW

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Re: Charging a 12V lead battery from a 18s system

Post by ZeroEm » Oct 19 2020 4:10pm

Sorry, my brain never seen winch. :oops: Yes they get power hungry when they get a load and start grunting. :flame:
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john61ct   100 GW

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Re: Charging a 12V lead battery from a 18s system

Post by john61ct » Oct 19 2020 4:45pm

Charging a battery properly requires a proper charger, whether DC-DC or AC-DC via an inverter.

From 60-90Vdc downwards requires an appropriate buck converter, target say 22Vdc

and you could feed that into a cheap solar controller.

Depends on the current you require, which in turn depends on your target bank Ah capacity.

Current limiting is not much of an issue with lead.


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Re: Charging a 12V lead battery from a 18s system

Post by john61ct » Oct 19 2020 4:48pm

Look at the larger Victron SmartSolar MPPTs, a 100/x 120/y or 150/z unit might just do the trick all on its own.

Most purpose-designed DCDC chargers max out around the 48V range.

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Re: Charging a 12V lead battery from a 18s system

Post by john61ct » Oct 19 2020 4:51pm


HrKlev wrote: a normal 10A 60-96vdc to 12vdc power supply
please link to examples of what you consider normal units available in that category.


HrKlev   100 W

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Re: Charging a 12V lead battery from a 18s system

Post by HrKlev » Oct 20 2020 12:37am

Yes, that Victron unit looks like exactly what I could need. It is a little out of budget atm, unfortunately. Would this aliexpress controller do the trick at the moment, you think?
https://a.aliexpress.com/_mOxiKFj
Im thinking I could just keep the original 12v system on the ATV including the 12V battery, and just use this to keep it topped up, like an alternator. The load port on the controller would be left unused.

I have been using this on an earlier ATV conversion:
https://www.ebay.co.uk/itm/362993387594
It supplies lights, horn, heated grips, etc with 12v. I have nothing on that vehicle that draws a lot of power, like a winch, though.
I have ordered one like this:
https://www.ebay.com/itm/133200542370
I want to replace it with. It has isolated 0v and looks a bit more rugged.
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HrKlev   100 W

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Re: Charging a 12V lead battery from a 18s system

Post by HrKlev » Oct 20 2020 1:00am

Actually the small Victrons was a lot cheaper than I expected. If I can keep the 12V battery in the system to take care of the power surge, I could get away with the 100/15 version I would think. And feeling a lot more safe than using the no-name aliexpress version.

Just to be sure; If I connect the solar input to the 18s battery with "unlimited" (compared to a solar panel at least) current supply, it is OK? It will limit itself to draw whatever it needs to charge the 12v battery with 10/15/30A (depending on version)? I would put a fuse between controller and 18s battery, of course.
DIY lightweight mid drive with torque sensing build thread: viewtopic.php?f=6&t=107622

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Re: Charging a 12V lead battery from a 18s system

Post by john61ct » Oct 20 2020 6:11am

A FLA battery cannot draw more than 0.20-0.25C even well depleted.

AGM maybe 0.6-0.8C, but not for long.

You may need to turn off the SC while running the winch though, or put in a fuse at around the SC max amps rating.

Victron allows for higher input amps, a bit anyway, but better to be conservative.

If you want "live" conversion, winch pulling current through the DCDC from the propulsion pack, you will need a converter rated for higher than those current levels.

And if the winch is pulling for more than a couple minutes, do not underestimate the Ah consumed.

A gasoline powered winch is worth considering for high weights, longer distances.

HrKlev   100 W

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Re: Charging a 12V lead battery from a 18s system

Post by HrKlev » Oct 20 2020 8:49am

OK, I think I will order a 100/15 version. I dont think the lights and other accesories will draw more than 4-5 amps, so 15 amps on the charger should be plenty to top up the battery after some winching. I will use a 3A fuse on the high voltage side. I found a datasheet that says "max PV short circuit current" 15A. Not quite sure what it means, but should be safe with a 3A fuse?

The winch will only be used whenever I get stuck, so not for long periods. Thats why I dont want to cash out for a big version.

I could just charge the 12v battery now and then, and use the DCDC powersupply for the accesories, but I know myself enough to know the day I need the winch the 12V battery will be dead.... I think this will be a much better "fit and forget" system. Thanks for helping me in the right direction!
DIY lightweight mid drive with torque sensing build thread: viewtopic.php?f=6&t=107622

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Re: Charging a 12V lead battery from a 18s system

Post by john61ct » Oct 20 2020 1:38pm

HrKlev wrote: datasheet that says "max PV short circuit current" 15A. Not quite sure what it means, but should be safe with a 3A fuse?
That amps rating means the input current is OK at 15A, lots of power since the input voltage is higher, up to (in theory) your maximum input voltage, W = V*A

Approaching that 100V limit on the input side can fry the SC pretty quick, so stick to say 90-95V max peak, even at low current levels.

The "real" limit is that the SC will not **output** much over 15A, so really there is no point in feeding much more power than what is required to get to that point (depends on the output voltage you set) V = W/A

The load (winch) is what determined the power drawn through the system, thus the amps, up to the 15A limit on the SC output.


> The winch will only be used whenever I get stuck, so not for long periods.

"Too long" could be 3 min, or 30sec or 6min

You need to measure the actual peak V&A with a wattmeter and compare to the 12V battery actual Ah capacity

assuming you disconnect the SC first to ensure the winch's draw doesn't overwhelm - that's where proper fusing will protect the SC, if you decide it's safe to allow it to support the battery, feeding an extra 15A from the high-voltage side.

Plus normal fusing against shorts, protecting the wiring from over-current.

Wiring should be sized for 20% above the max current you think it will ever see.

With the SC on fulltime while the vehicle is operating, it is powering the accessories, the battery will just sit floating, say 13.8V or whatever, at 100% SoC

Only after it gets somewhat depleting will the SC kick into higher voltage, actual charging mode at say 14.6V or whatever, and for lead 7+hours is required to get back to Full for longevity - so plug it in back home overnight as well if the high voltage side gets shut down.

Isolate all batteries when the vehicle is not in use.

Lead needs periodic topping up to stay at 100%

Conversely, LI needs to be stored at low SoC

HrKlev   100 W

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Re: Charging a 12V lead battery from a 18s system

Post by HrKlev » Oct 21 2020 12:13am

john61ct wrote:
Oct 20 2020 1:38pm
[
The "real" limit is that the SC will not **output** much over 15A, so really there is no point in feeding much more power than what is required to get to that point (depends on the output voltage you set) V = W/A

Alright, I see. Thats how I was hoping it would work. So in a worst case scenario it charges 14.4V*15A=216W. Absolute minimum cell voltage on the main battery before BMS kicks in will be 3V, so 54V in my 18s system. That would be mean 216W/54V=4A draw. Plus efficiency loss and some headroom, I will use a 5A fuse on the high voltage side. Max charge will be 4.15V*18=74.7V, so there are some headroom for max voltage as well.

As long as the SC limits itself to 15A, I was hoping that it shouldnt be a problem just leaving it on while using the winch...? Putting a switch inline is no problem, but it will be used a lot by my father as well, and I know he will forget that switch.... :p

I will test the winch and size the battery accordingly. I have a pretty big one I am considering using, as I think the ATV could need some extra weight up front as well.

Victron will be ordered today :)
DIY lightweight mid drive with torque sensing build thread: viewtopic.php?f=6&t=107622

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Re: Charging a 12V lead battery from a 18s system

Post by john61ct » Oct 21 2020 12:41am

I honestly have no idea how the SC would handle a huge overcurrent draw event with an "unlimited" input available current.

It is designed for panels, completely different source.

Personally if you can't test the winch for max draw (why not?) then I'd advise some sort of A/B switch / relay so enabling the winch cuts off the charging (at SC input), disabling the winch re-enables charging.

HrKlev   100 W

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Re: Charging a 12V lead battery from a 18s system

Post by HrKlev » Oct 21 2020 12:48am

OK, I see. I'm away for work for a couple weeks, so I cant test it right now, and I want to order parts so I can start when coming home. Experience from another winch I have, I would guess at least 100A max. I will find out what happens. It would surprise me if the Victron unit doesnt have some sort of limiting on the charge current.
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Re: Charging a 12V lead battery from a 18s system

Post by Hillhater » Oct 21 2020 7:01am

Any charger MUST have charge current control...by definition you cannot have a CC/CV charge algorithm without current control and limiting.
I would also expect any charger worth buying would also have ADJUSTABLE charge current settings.
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HrKlev   100 W

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Re: Charging a 12V lead battery from a 18s system

Post by HrKlev » Oct 21 2020 3:22pm

Yeah, I would think so. I'll try to leave it connected when winching, so I guess I will find out. Just need to get on with the conversion and do everything else first :) Think all parts are on their way now, this was the last major piece of the puzzle.
DIY lightweight mid drive with torque sensing build thread: viewtopic.php?f=6&t=107622

john61ct   100 GW

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Re: Charging a 12V lead battery from a 18s system

Post by john61ct » Oct 21 2020 7:47pm

Yes I checked and Victron SS units do seem to let you derate current output.

However again, I would test AMAP, not vouching for a unit doing well over time if a say 600A load is trying to "pull through" the SC from an upstream bank happy to supply that.

Fuses are relatively cheap as a fallback.

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