Home made DC-DC battery charger

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PillowSmuggler
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Home made DC-DC battery charger

Post by PillowSmuggler » Wed May 24, 2017 9:59 pm

This is probably one for XTC but it's open to all comers!

Given the findings that the smart alternators in our cars are simply not designed to charge additional batteries fully, i was wondering about installing a dc-dc battery charger to keep my aux battery full (instead of 80%).
Or at least I was until I saw the price - £160+ :shock:

So, time for homebrew :)

What if I used a boost buck converter like over in Gary's laptop thread to raise the car volts to say 25v and then fed that into a MPPT solar panel controller to recharge the Aux battery? Such controllers usually employ multiple charging stages similar to a CTEK to get the max possible into the battery. Apart from being ugly, it will be a lot lot cheaper!

Is that pure mad, or might it just work?


Regards, Dom.
2007 HSE Auto TDV6
Snorkel, Cyclonic air filter, GNVP Rock Sliders
78Ah Aux, Split charge, FBH Timer/Remote, Mantec sump plate
LED: Strobes, interior, camping, brake, running, sides
New Oil Pump, using 2SO at 1:400.

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xtc
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Re: Home made DC-DC battery charger

Post by xtc » Wed May 24, 2017 10:47 pm

I like it! Can only not work at worst :mrgreen:
Maybe this instead of open PCB ?
http://www.ebay.co.uk/itm/DC-DC-Convert ... SwnbZYDxjy" onclick="window.open(this.href);return false;

Only worry with either up converter would be limiting the current so as not to damage it. The battery if flat would draw all it had
to give, so maybe some form of current limiting between the converter & regulator would be needed? I think the PCB had a pot to
limit the current so may be better than the block. I'm sure I have all 3 bits in various boxes if you want to play!


Off Roading is OK.... But I'd rather be Boating!

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Re: Home made DC-DC battery charger

Post by PillowSmuggler » Thu May 25, 2017 10:29 pm

Thanks Rich :)

This open module has current adjust (http://www.ebay.co.uk/itm/122255409935" onclick="window.open(this.href);return false;) so I think that'd be the best one to start with - I agree that allowing the system to hit max current is somewhat nuts, and frankly I don't want to push that much power into the battery as I'd prefer a slower charge that leads to battery longevity. Being able to adjust the output current also essentially controls the input current, and with a 150A alternator whizzing away I'd be less concerned with stealing 15A for the charge circuit and more concerned about the up converter taking that much power and dumping all that heat from the inefficient conversion LOL.

I have a charge controller already thanks Rich, appreciate the offer of the bits but if I'm going to let the magic smoke out of some electronics I'd rather they didn't belong to someone else! :lol: :lol:


Regards, Dom.
2007 HSE Auto TDV6
Snorkel, Cyclonic air filter, GNVP Rock Sliders
78Ah Aux, Split charge, FBH Timer/Remote, Mantec sump plate
LED: Strobes, interior, camping, brake, running, sides
New Oil Pump, using 2SO at 1:400.

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Re: Home made DC-DC battery charger

Post by Mingee » Thu Oct 26, 2017 10:55 am

What ever happened to your project? Did you pull it off?



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PillowSmuggler
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Re: Home made DC-DC battery charger

Post by PillowSmuggler » Thu Oct 26, 2017 9:51 pm

I did indeed. I think the principal will work as for a moment I did have a charge voltage heading into the battery, but I need to be less clumsy and remember to set the multimeter back to volts before i try and read the volts across some components :oops: Sadly I think the component I was measuring simply couldn't take the current that should never have reached it; I don't think it was even close to enough current to blow a fuse, just enough to cook the IC.
I now have a nice Chinese DC clamp ammeter to stop that happening again and I have a new charge controller on order to replace the one I smoked - thanks for the reminder, it's way overdue and I need to chase it or get it resent.
Once all the bits are back together I'll take some proper readings and report back on the currents, voltages and losses and see if I can find what the best step-up voltage is to make it all work without working too hard.


Regards, Dom.
2007 HSE Auto TDV6
Snorkel, Cyclonic air filter, GNVP Rock Sliders
78Ah Aux, Split charge, FBH Timer/Remote, Mantec sump plate
LED: Strobes, interior, camping, brake, running, sides
New Oil Pump, using 2SO at 1:400.

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PillowSmuggler
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Re: Home made DC-DC battery charger

Post by PillowSmuggler » Tue May 22, 2018 7:26 pm

Well, I finally got some time, got some new bits, and then smoked some more bits (primarily as the instructions didn't match the product and in one case was a voltage reducer not booster) , so I got some better bits and it seems like the principal is at least sound.
My booster is good for up to 10A at up to 40V output, and has separate current and voltage adjusters.

First off, using a Solar Charge controller to look after the charging :
IMG_20180521_172048 - Copy-800x600.jpg
Blue volt meter is monitoring the donor battery. It's a 85Ah Leisure battery and has a partial charge.
Red Voltmeter shows the intermediate voltage after the step-up voltage unit
Yellow Voltmeter shows the voltage of the battery being charged.
As you can see there's 3.5 Amps at 14.4 volts being generated by the Boost converter. The red LED on that is because I set up current limiting and it's limiting to 3.5A.
The input to the unit is 4.5A at 12.1V ~= 55Watts
The Output from the unit is 3.5A at 14.4V ~= 50.5W
So losses in the up-converter are around 10% or 5W and it was indeed getting warm though still touchable.
The Solar Charge controller was dropping the input from 14.4 to 13.7, but there was only 3A getting to the battery, and at 13.7V it was in for the long haul as far as charging went. That represent 20% losses, all as heat, and that was getting very hot.

So next I cut out the middle man:
IMG_20180521_173439 - Copy-800x600.jpg
This seemed to work much better, with a slightly higher voltage (but below the battery charge limit of 14.4; I'll try that later). Losses were in the order of the 10% on the up-converter, but current reaching the battery was much larger at 4.6A
So, only charging at 5AH at the moment, but it's getting there!


Regards, Dom.
2007 HSE Auto TDV6
Snorkel, Cyclonic air filter, GNVP Rock Sliders
78Ah Aux, Split charge, FBH Timer/Remote, Mantec sump plate
LED: Strobes, interior, camping, brake, running, sides
New Oil Pump, using 2SO at 1:400.

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