DC-DC Chargers - When do they stop charging?

[meetyg]

Hi.
I have a general question about DC-DC chargers. I'm guessing that not all products are alike, but probably work the same:
When does a DC-DC charger stop charging the battery to be charged? Is it based on voltage only, or based also on current?

For example, if the DC-DC charger sees that the battery is at (or above) the charging voltage (for that chemistry), will it stop supplying power all together, or will it continue to supply power but at a lower current?

I was looking at this product for a little experiment I am doing with my EV:

DC to DC Battery Chargers|12V Battery DC to DC Charger-OLYS

DC to DC battery chargers designed to get the most out of your battery powered vehicle，keep your secondary battery charged whilst driving

www.olyssolar.com

I want to try to offload the auxiliary loads on the 12v system of my EV by supplying power using some solar panels, and another battery pack (please don't comment on the costs/feasibility, I have already gone through all that on an EV forum). The goal is to use less of the high-voltage battery and more from solar. But because space is limited for panels, I am planning to charge another battery (say a Lifepo4) while the car is parked using solar, and then while driving supply that power to the 12v circuit, in parallel with the existing 12v battery (some form of Lead-Acid).
The problem is I'm afraid that because the EV has a DC-DC converter on board (taking power from the HV battery and supplying it to the 12v battery and loads), the DC-DC charger ( the one I want to add) will see that the 12v circuit already has high voltage (around 14.4v) and won't supply the 12v circuit with power from my lifepo4 battery.

Does this make sense?
My other option is simply to get a generic DC-DC boost converter with adjustable output voltage and current, which I will set with a voltage high enough to overcome the on-board HV DC-DC converter (let's say 14.6 volts).
Thanks in advance....

 

[Short_Shot]

What car do you have?

Doesn't your EV use a "standard" 12v battery for everything other than drive? That link even states that's what it's for do its not like it's a new scenerio. If so...

All you need to do is put a charge controller on that and send it. The vehicle's built in DC to DC unit should have a threshold voltage that's below 100% charge and the solar will behave more like a glorified battery tender with the car kicking in at higher loads/low solar.

Do you understand the behavior of the cars HV DC to DC converter? If so, describe it?


If your EV DC system voltage is at 14.4 (or whatever charge voltage) from the solar charge controller then you wouldn't need to use the DC to DC charger since it's already charging the system.

If you have a load that exceeds the solar output then the voltage from the SCC would drop until the battery is being drawn from.

At that point your solar is simply insufficient and the car would take over.

 

[meetyg]

Basically yes, it has a 12v for all the common uses a gasoline car has.
You are also correct that I could have just put an SCC in it, and have the solar charge it up, either when stationary or while driving.

The thing is that I'm planning to put around 200w of solar power (peak), which might offset some of the load, but the car uses around 500w minimum when on.
My plan is to add another small battery to act as a buffer, that will be charged up by the solar when the car is parked.
Then when driving, that buffer battery will supply more of load than just the solar panels.
Of course when the buffer battery reaches the end of its capacity, the car will still use the onboard DC-DC converter and conventional 12v battery.

The onboard DC-DC converter has two functions: 1. supply the auxiliary power, 2. recharge the 12v (like an alternator).

Having that the standard 12v battery is used very lightly in an EV, it charges up pretty quickly, and from then on the DC-DC converter supplies all the load.
So there is not much point of just charging the standard 12v, as it will be almost fully charged anyways.
I just want to "trick" the onboard DC-DC converter to think that it has lesser load (by supplying it from the buffer battery) and therefore work less, and save some of the HV battery capacity.

 

[meetyg]

Basically, this guy is doing something similar, but he is bypassing the standard 12v system altogether, by using a solar charged battery bank he has in his trunk anyways.
My plan isn't to bypass the current system, rather to supply it in parallel.
But he explains the motivation better:

 

[curiouscarbon]

Having that the standard 12v battery is used very lightly in an EV, it charges up pretty quickly, and from then on the DC-DC converter supplies all the load.
So there is not much point of just charging the standard 12v, as it will be almost fully charged anyways.
I just want to "trick" the onboard DC-DC converter to think that it has lesser load (by supplying it from the buffer battery) and therefore work less, and save some of the HV battery capacity.

cool, i’m trying to implement VIPV vehicle integrated photovoltaic also

the PV->BufferBattery->DCDC->Vehicle12VBattery idea seems sound to me.

as you say, the aux battery already has a “Buffer Battery” which is the HV traction battery in a sense?

anyways, to augment car 12V battery power with solar i would do something similar to what you describe.

panels charge lifepo4 buffer battery over course of day and then serve as a regenerating supplement to the main HV traction battery

during idle the buffer dcdc can keep the 12V battery at full charge so that the HV battery doesn’t get drained as much. that makes sense to me.

would need to measure and stuff to see what actually happens. depends on vehicle charging algorithm too maybe.

fun to simultaneously monitor 12V voltage over course of day and note actions see correlation just kind of randomly thinking out loud.

good luck with your vehicle integrated photovoltaic project!

 

[meetyg]

cool, i’m trying to implement VIPV vehicle integrated photovoltaic also

the PV->BufferBattery->DCDC->Vehicle12VBattery idea seems sound to me.

as you say, the aux battery already has a “Buffer Battery” which is the HV traction battery in a sense?

anyways, to augment car 12V battery power with solar i would do something similar to what you describe.

panels charge lifepo4 buffer battery over course of day and then serve as a regenerating supplement to the main HV traction battery

during idle the buffer dcdc can keep the 12V battery at full charge so that the HV battery doesn’t get drained as much. that makes sense to me.

would need to measure and stuff to see what actually happens. depends on vehicle charging algorithm too maybe.

fun to simultaneously monitor 12V voltage over course of day and note actions see correlation just kind of randomly thinking out loud.

good luck with your vehicle integrated photovoltaic project!

Thanks

Well yes there is the HV traction battery, but is alot hassle to charge it. You would need a DC-DC boost converter to convert from 12v to around 400v.
Working with those voltages and tapping into the HV battery is something I think should be done by proffesionals only, or at the manufacturing stage.

Offsetting the auxiliary loads should be alot easier and safer.

What are you working great on? I would like to know more, maybe get some ideas.

 

[curiouscarbon]

so far the routes i’m focusing most engineering effort on are: (input->output)

1) DCPV->MPPT->LFP->DCDC->LeadAcid12V
2) DCPV->MPPT->LFP->Inverter->AC mobile EVSE->car charge port->HV battery

first one for mainly stationary use
second one to actually add travel range back

 

[meetyg]

so far the routes i’m focusing most engineering effort on are: (input->output)

1) DCPV->MPPT->LFP->DCDC->LeadAcid12V
2) DCPV->MPPT->LFP->Inverter->AC mobile EVSE->car charge port->HV battery

first one for mainly stationary use
second one to actually add travel range back

Cool.