CC CV power supply as charger in car

[disco stu]

Hey all. New here, and keep coming across the forums when looking for info.

Basically, looking to set up a small simple battery set up for the car to power fridge while away camping. Not exactly made of money so trying to keep things on the lower cost side, and my needs are quite simple as I really only need to power the fridge for a few days. Charging at this stage is intended to just be the alternator while driving around between fishing spots etc.

Cost of DC to DC chargers that everyone recommends is around the same as I'm looking at for the battery itself. It all just starts to look too hard and expensive. Most other people in my situation seem to throw thousands at just powering a fridge. Paid $700 for the 4wd if that gives some context on my budget.

I'm not completely new to lithium type batteries, using them a fair bit in the past in the hobby type area, making up drill packs etc etc. Looking at something like a 40-50ah LiFePO4, may make the pack up myself but that's a different conversation.

I know constant current constant voltage is used commonly with bench power supplies to charge cells and packs. Finding a boost CC CV power supply to run on 12v dc is quite cheap, much much cheaper than DC DC charger and plenty of options in the amp size I'm looking for.

Is there a good reason why I can't just use one of these boost cc cv converters set to 14.6v and say 10a to charge while driving around?

My one concern is that they likely won't drop the current completely once battery is full and continue to pump in 200ma or so. I've been thinking about coming up with some way to either open a relay once current drops under a set amount, or just power a buzzer to alert me switch off the charger. Or is this not even a big issue and battery will absorb that?

Appreciate any advice you can offer

Cheers, Stu

 

[mikefitz]

Set your power supply to 13.8 volts , that's all you need.

 

[disco stu]

Thanks for reply.

I take it from your comment that there's no issue with my idea then. At 13.8V would there then be no issue with residual current going into battery of around 200mA?

 

[RCinFLA]

If you know what you are doing it is fine. CV/CC power supply is an adjustable voltage capped, constant current source.

Most common error is folks don't realize how CV/CC works. They don't realize the voltage will drop to present battery state of charge voltage to maintain the constant current.

When they connect it to battery and power supply voltage drops below what they want for final voltage they try to crank up voltage on power supply. This results in overcharging as the battery gets close to full charge the power supply voltage rises beyond the max charge voltage limit on battery, damaging the battery.

Many of the cheap units have low voltage/current feedback control gain. What this means, as voltage rises and approaches the capped voltage limit setting, the current limit begins to drop off prematurely.

A good CV/CC power supply will maintain the set constant current right up to the point where it hits the limit voltage setting. Cheap CV/CC supplies can start to taper off constant current when voltage is 0.5v below voltage limit setting on supply. It still works but it can extend the battery charge top off time.

Power supplies with push button settings often switch display value when going into CC or CV adjustment mode to show the actual limit settings, then timeout and drop back to displaying actual current and voltage of output terminals. This makes them more user error proof.

 

[disco stu]

Thanks. Good info.

I'm good on the battery voltage vs power supply at least, but still appreciate you pointing that out.

Any issue with small current still going in if the power supply doesn't drop right down to zero current? I figure if fridge is on it will draw down more than that amount, but could be 15 min or so where battery is full and small amount out current going in

 

[disco stu]

Had a thought for simple charging disconnect.

Seems to be plenty of current monitoring relays around, but from what I can gather they're all aimed at shutting off over current situations.

My thought was to use one of these powering a normally closed relay, seeing they will output power when under the threshold current. Set it to whatever low current you want charging to switch off at. Current drops under that value, current monitor allows power through, NC relay is powered on opening contacts and stopping charge current. Could throw a buzzer or LED on to be powered on at the same time.

Will depend on the specific unit if it has a parallel power output or has to have all current flowing through it, but just a thought I had