Dc-dc off grid chargers

[Chris200]

Hi all, I am looking into of grid charging options and wondered if any one could offer a little advice..

I live off grid and use solar as my main energy source. So I am well aware of the inefficiencies of using inverters to charge low voltage DC appliances. With that in mind I have been looking around for a dc-dc charger.... Very little available. I have however found dc-dc converters on eBay..

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Has anyone tried these? And if so what parameters did you set it at? I have a 48v 17 5 ah battery. So assume 54v @ 2a would be ok... Would the bms then control the charging?

Secondly I have a victron mppt solar charge controller that can be set for different battery set ups. I.e you can tell the controller what voltage/type your battery bank is and it charges appropriately. I am wondering if it would be crazy to simply wire a battery connector directly to a solar charge controller and and set it to 48v lithium ion... Any opinions much appreciated

 

[wheeliepete]

I've used a cheap Chinese MPPT controller to charge my bike battery pack. As long as it has cc/cv li-ion charge function and variable voltage output, it should be fine.

 

[vfr400]

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[Chris200]

I've used a cheap Chinese MPPT controller to charge my bike battery pack. As long as it has cc/cv li-ion charge function and variable voltage output, it should be fine.

Thanks! That's a start

I some to my solar supplier about victron mppt controllers and he said it was important that the battery bms controlled the charging process... My understanding was that all the magic happened in the charger not the battery. Am I wrong? I have emailed en3ev to find out what the bms I'm my battery is doing but no reply as yet...

 

[vfr400]

The charger just needs to give a constant 42v (for 36v battery). The BMS in the battery does the rest.

 

[Chris200]

The charger just needs to give a constant 42v (for 36v battery). The BMS in the battery does the rest.

Sweet, so for my 48v battery I need a constant 54.4v? I have been looking at mppt buck boost controllers. So rather than having more panels you have a controller that jumps up the voltage... As well as charge controlling. That said maybe I should just use a buck booster with current 12v set up??

 

[vfr400]

The important thing is that you can set the output to the correct charge output and current for your battery. Most batteries should be charged at around 0.2C, which for batteries in the range 10Ah to 15Ah would be 2 to 3 amps.

 

[WheezyRider]

If using a DC/DC converter for battery charging, it's important to set the constant current control (CC) to a suitable level for your battery pack. If it is set too high, it could do damage. Typically, (as VFR says) for a 10 to 15Ah/36V pack two to three amps is a good target. When the converter is charging, it will reach the set current and reduce the output voltage as required to do this.

You could attach a dummy load to the converter and measure the current with a multimeter on the high current setting (usually 10A).

For the final part of the charge you need to set the constant voltage control (CV) to 42V. This stops the converter from sending too high a voltage to the battery. During charging, the voltage will increase until it reaches 42V. This can easily be checked using a multimeter set to volts. Bear in mind that many cheap multimeters can easily be in error by 0.5V, so if you are not sure on the meter's accuracy it's a good idea to turn it down just below 42V. Of course, this is for a 10 cell pack, so change the voltage as required for what is the nominal voltage of your pack.