Charging lead acid from lithium no alternator

[GHen]

Please let me know if there is already a post that covers the subject, although I’ve been searching.

5th wheel trailer has 400ah of 12v lead acid with 300w of solar. System works great. But a few days of camping with no sun or lots of trees requires some recharging. I have a portable generator but also have a 200ah 12v lithium portable power station.

What’s the best way I dump the power from my Lithium power station into my lead acid battery bank.
1) I know I can use an inverter and battery charger from the power station, maybe 80% efficient..
2) direct connect the batteries, This probably would work until the batteries are even.
3) how about a 12-24v boost converter to MPPT. Seems about 90% efficient.
4) dc to dc charger, but I don’t have an alternator connection that they seem to need.

Think of it as this. Trailer is in a remote area. I take the power station to town or to sunny location charge it up and then dump it al the powerl in my lead acid back at camp. Like going to town to fill a gas can.

Any suggestions are appreciated.

 

[MaikaiLifeDIY]

I think option number two is probably gonna be your best cause there’s no conversion loss. However, once the voltage of the lithium drops to the equivalent of the rest of the battery bank, that’s all the power that’ll be pushed. The good news is the lithium don’t drop in voltage until they get really close to the end of their capacity so most likely they’ll remain at a higher voltage all the way down to 80 or 90% discharge then your lead acid batteries.

 

[grizzzman]

I would suggest a buck/boost DC to DC charger for this project that is adjustable.

 

[GHen]

Thanks for the replies.

The direct connection interest's me. If the lead acid is low, like 12.2v or so, would I need to be concerned with a rush of current between the two? If so, how many amps could the rush be? I would need to size the cables properly. I may test this while checking the amp flow and connection temperatures.

As far as the buck/boost dc to dc charger, Are you referring to the buck/boost converters or is there a model that actually controls the charging function also. Or do I still need a MPPT to control the charging?

 

[grizzzman]

Thanks for the replies.

The direct connection interest's me. If the lead acid is low, like 12.2v or so, would I need to be concerned with a rush of current between the two? If so, how many amps could the rush be? I would need to size the cables properly. I may test this while checking the amp flow and connection temperatures.

If your FLA is at 12.2 and your LFP is at 13.4/5 the inrush will be large. As a example when I connect my LFP to my FLA I see a short temporary inrush of over 100 amps and this is when the batteries are close in voltage. I use a 500 amp contactor to connect the two banks. Of course the resistance path between the banks will play a roll in this.

As far as the buck/boost dc to dc charger, Are you referring to the buck/boost converters or is there a model that actually controls the charging function also. Or do I still need a MPPT to control the charging?

As a example have a Renolgy 40 amp DC to DC charger. During testing connected to 14 volt output truck was pulling 50 amps and the output was 14.7 at over 39 amps. I do not know if this is true to all MPPT's but Victron requires 5 volts over battery voltage to start.

 

[GHen]

Great info, thanks.

On your Renogy DC to DC charger example, do you use the D+ connection to detect that the alternator is running? That’s my biggest question, if I can use a DC to DC charger without an alternator connection.

 

[pollenface]

Using a current limiting boost converter allows you to precisely control how much you are drawing from the donor battery. I also use a shunted inline monitor to gauge this when making adjustments.

I have one of these, it seems robust (with added fan)

 

[wme]

One answer...

Not a fast charge but a trickle

 

[grizzzman]

Great info, thanks.

On your Renogy DC to DC charger example, do you use the D+ connection to detect that the alternator is running? That’s my biggest question, if I can use a DC to DC charger without an alternator connection.

Yes you can. As a example my DC to DC is set up as a jumper with 20 foot cables. I simply connected a fuse holder to the power side. Pull the fuse, connect the cables and install the fuse and the setup is off and running.

 

[GHen]

T

One answer...

Not a fast charge but a trickle

WME, thanks, this idea would work well also. I could connect and trickle charge all night long. But, I thought a solar charge controller needed a higher voltage as another person had noted. That’s why I was thinking I needed a boost converter between the lithium and the SCC. I need to do more research.

 

[Vigo]

You could make your own 'dc-dc charger' using a boost converter and a cheap PWM solar controller, but... You have to kinda use the cheapest of the cheap before it just makes more sense to buy an actual pre-built charger.

So for example you could use a $15 boost converter and a $15 pwm solar controller to make a homebrew DC-DC charger but by the time you're spending $50 or $60 you coulda just bought a 'real' DC-DC charger for $64 such as this one: DC DC Charger