I have a battery charger based on a server power supply (link to my model). It takes 240V mains voltage as the input and produces an adjustable CC/CV output to charge an LiFePO4 battery. I usually set it to 14V, 50A to charge my 280Ah battery.

I also have an electric car that charges its "starting" battery with 12.4 to 14.4V DC. Rumoured output of the onboard DC/DC converter is at least 1000W.

Is there a way to modify my charger to take 12V DC input instead of 240V AC input? Maybe by connecting my 12V to somewhere internally between the AC and DC sections of the board? That way I'd just be using the charger to provide current limiting between the car and the battery.

I know I can connect an inverter across the car and plug the charger in to the inverter, but I'm trying to avoid the 15% conversion efficiency losses of having the inverter in there, hence the question. I can't just connect the battery to be charged across the car, because it will likely draw more current than the car can supply and blow something up (I don't believe the car does CC charging of its starting battery, and at any rate it's designed for a 60Ah lead-acid starting battery).

 

If you start with 12V and want 14V, I don't see how you can achieve that without the use of an inverter, which you clearly wish to avoid doing.

 

How do you control the charge profile while using your 240 volt power supply? Do you monitor battery temperature? This is especially important with lithium based batteries.
You mention that you want to modify the 12 volt charger from your electric car to charge your LiFePO4 battery. This means you are still losing to efficiency by charging your LiFePO4 from your car main battery. How do you charge the main battery of your car and can you use that source to charge your LiFePO4 battery?

 

If you start with 12V and want 14V, I don't see how you can achieve that without the use of an inverter, which you clearly wish to avoid doing.

The car actually outputs a charging voltage, it varies between 12.6V and 14.4V depending on current flow, so I wouldn't be boosting voltage, just lowering it.

 

How do you control the charge profile while using your 240 volt power supply? Do you monitor battery temperature? This is especially important with lithium based batteries.
You mention that you want to modify the 12 volt charger from your electric car to charge your LiFePO4 battery. This means you are still losing to efficiency by charging your LiFePO4 from your car main battery. How do you charge the main battery of your car and can you use that source to charge your LiFePO4 battery?

The charge profile for lithium iron phosphate batteries is very simple, just a constant current (equal to 1x cell capacity) up to 3.6V/cell then hold at 3.6V until current decays to 5% of cell capacity. In my case it would be 280A and 14A respectively, though my charger can only manage 100A maximum so charging takes a bit longer.

Temperature monitoring, individual cell voltage monitoring, balancing and many other protection functions are done by an active BMS within the battery - all that's needed externally for charging is the CC/CV supply.

The idea is that in a power cut I use my LiFePO4 to power the house via an inverter, then if the power cut gets prolonged I use the EV to supply more power - I was just looking for a potential way to efficiently charge the LiFePO4 battery without incurring inverter losses.

 

Renogy has chargers like that.

https://www.renogy.com/battery-chargers/

 

Yes, the way to do it is to forget the 240 volt charger and use a step-up switcher circuit. Boosting 12volts to 14.5 volts should not be an issue. Designing the circuit using one of the help files at the Texa Instruments website should not be terribly difficult. But building an actual switcher regulator will be a challenge if you do not adequately understand the circuit and it's operation.

 

Yes, the way to do it is to forget the 240 volt charger and use a step-up switcher circuit. Boosting 12volts to 14.5 volts should not be an issue. Designing the circuit using one of the help files at the Texa Instruments website should not be terribly difficult. But building an actual switcher regulator will be a challenge if you do not adequately understand the circuit and it's operation.

Thank you, yes that would be the way to go, I'm just a bit lacking in the power electronics design department . We're talking 12V 100A so needs quite beefy components and a good understanding to actually build something reliable and safe for the battery and the car.

 

Wait a while there!! WHAT SORT OF BATTERY CHARGING WOULD YOU BE DOING WITH 100 AMPS??? There is a whole lot of information missing there. Like how a "starter battery" needs 100 amps to charge. What are you trying to achieve?? I seem to be a bit lost here.

 

Wait a while there!! WHAT SORT OF BATTERY CHARGING WOULD YOU BE DOING WITH 100 AMPS??? There is a whole lot of information missing there. Like how a "starter battery" needs 100 amps to charge. What are you trying to achieve?? I seem to be a bit lost here.

Fair comment. I'm charging a 280Ah 4s LiFePO4 (nominal voltage 12.8V). According to the datasheet it can be charged with currents of over 200A, but my charger can only manage 1.2kW, corresponding to 85A @ 14.1V.

I use the 280Ah battery to run the house during power cuts. In case I completely exhaust this backup battery during a power cut, I'm looking for a way to charge the battery using the energy in my electric car's battery, but doing it efficiently. I could just connect my inverter to the car, then the existing LiFePO4 charger to the inverter, and it would work to charge the 280Ah battery, but at 85% efficiency for the inverter and then 85% for the LiFePO4 charger, I'd be throwing away a lot of energy - hence the question on whether it would be possible to modify the charger to take 12V input [ideally I'd tap into the car's 400V traction battery directly and avoid the losses from the onboard DC-DC converter, but that's way beyond the level of risk I'm willing to take].

I understand there are commercial devices (Renogy chargers, DC-DC converters) that will do the job, but they are all very pricey (€250+).

 

OK, I see what the goal is. I also see an efficiency issue, which is that battery charging, especially at high rates, generates a fair amount of heat, and every bit of that heat is wasted power. That is rather inescapable.
What could be more efficient is a load sensing inverter that would only consume the power needed to supply any actual loads that were present. In fact, that sort of inverter system would be the best option for both battery supplies.

In addition, taking power from the vehicle battery system assumes that there will be no need to use the EV during or soon after the power outage. That may be an unfortunate assumption if it becomes incorrect.

 

OK, I see what the goal is. I also see an efficiency issue, which is that battery charging, especially at high rates, generates a fair amount of heat, and every bit of that heat is wasted power. That is rather inescapable.
What could be more efficient is a load sensing inverter that would only consume the power needed to supply any actual loads that were present. In fact, that sort of inverter system would be the best option for both battery supplies.

In addition, taking power from the vehicle battery system assumes that there will be no need to use the EV during or soon after the power outage. That may be an unfortunate assumption if it becomes incorrect.

Thanks for the replies. Not sure how a load-sensing inverter works, but if it's intended to lower standby power when not in use, that's not really a factor because I manually switch on the inverter and the background loads (internet modem, fridge etc) are 60W or more so I don't know whether it could standby effectively. My current 2.5kW inverter uses about 1.2A @ 12V when idle (no load). Using the car won't be such an issue because generally the power cut would be affecting a small area (for example the area covered by one substation) so I could easily drive the car a short distance to a public fast charger unaffected by the outage. It's rare for us to have a wide-area power cut where that won't be possible, but given that I have 3.5kWh in my LiFePO4 battery, and two electric cars holding 40kWh and 24kWh and that the whole house normally uses 7 to 12 kWh per day, that should be a good week of power backup if needed.

 

Load sensing inverter controls are not new at all, and so there should be information available, although I have not looked for it. And a current challenge with public fast chargers that I have read about is the lines that form and take time to move.
The most efficient system to charge from the 12 volt electric car batteries would be to use a boost type switcher power supply to directly rais the voltage from 12 volts to 14 or so volts. Charging at a lower current will be more efficient as it will producwe less heat and less voltage drop in the connections. Also, a lower current converter will cost less and be easier to build.

 

Is there a way to modify my charger to take 12V DC input instead of 240V AC input?

I would say not. Its 240V input will likely be rated for ~5A, so asking it to handle ~50A or more instead would probably require upgrading the semiconductors, the high frequency transformer, chokes, capacitors etc.
What is the Ah capacity of the 12V 'starter' battery?

 

I would say not. Its 240V input will likely be rated for ~5A, so asking it to handle ~50A or more instead would probably require upgrading the semiconductors, the high frequency transformer, chokes, capacitors etc.
What is the Ah capacity of the 12V 'starter' battery?

The starter battery in the car itself doesn't have an Ah label but judging by the size I'd guesstimate somewhere between 60-80Ah, and it's a lead-acid flooded (non-deep-cycle) battery.

The battery I intend to charge is a 280Ah LiFePO4.

I wasn't hoping to go via the 240V input stages, I was hoping to be able to tap into somewhere in the middle, in the part of the charger where it creates 12V (actually 14V). Maybe the transformer output. Then I'd inject my own 14V DC source from the car, and let the charger's DC section merely do the current regulation. But I am starting to think that the charger does its DC regulation by varying parameters (PWM, duty cycle etc) in the AC section - in which case my plan won't work because I'm bypassing the AC section so the charger has nothing to control to regulate the output current.

 

Tapping into the circuit of an existing charger would need to be evaluated by those able to understand correctly just what the circuit is doing. That is not a task for those who would be guessing.
And a caution now is that while a very recently charged lead/acid battery may show 14 volts, that voltage will very rapidly drop to 12 volts under much load.
An additional caution is that the charge providing circuit must be a higher voltage than the circuit receiving the charge.
So if the TS is able to provide a circuit schematic of the charger they want to utilize for charging, then advice not based on guessing can be provided.
Also, a caution that transformers do not usually work with DC.