Can I connect the new Dometic PLB40 to a YellowTop?

[MattJ]

I connected the charging port of the new Dometic lithium battery (PLB40) to my auxiliary battery (Optima YellowTop) and then connected the output of the Dometic battery to my fridge. I was careful to make sure the power cable is secured so that it can't get pinched in the fridge slide (have heard some stories about that). With this configuration, the PLB40 battery pulls power from the YellowTop until it is depleted. Then the PLB40 powers the fridge (up to a full week) until the PLB40 is depleted, too. Then when I start the Jeep using the cranking battery, the auxiliary battery recharges (via the Genesis solenoid system) and the whole cycle begins again. Any issues with this set up? I found an article that says different types of batteries can't be connected in line, but I'm only connecting the charging port of the PLB40 to the YellowTop. And my understanding is that the YellowTop is a deep-cycle battery made to be drained and recharged up to 400 cycles.

Thanks!

https://www.optimabatteries.com/en-us/experience/2014/04/can-i-connect-dissimilar-batteries-parallel

https://www.dometic.com/en-us/us/pr...atteries/dometic-plb40-_-197468#documentation

 

[john61ct]

The LI batt has higher resting voltage, so if energy efficiency is a concern, they should only be connected while charging.

An A-B switch can direct which feeds the fridge.

If you charge LI at the Optima setpoint, you will reduce its longevity.

Best to use a user custom setpoint adjustable DCDC charger.

 

[MattJ]

OK - the results are in! Let me know what how I should interpret what happened:

1) I charged the PLB40 at home with the AC adapter to 100% and measured its voltage at 13.66v.

2) I plugged the PLB40 into the Dometic fridge and let it run for four days, checking the voltage a couple times. At 44% the PLB40 was at 12.79v, 35% was 12.69v, and 6% was 10.99v.

3) At 6% and 10.99v, the fridge had shut down and was blinking an error code (presumably a low-voltage cut-off).

4) I disconnected the PLB40 from the fridge. I measured the resting voltage of the YellowTop at 12.65v. Then I connected the charging port of the PLB40 to the YellowTop using the auxiliary cable that I usually power the fridge with. So the two batteries were not connected in parallel, rather the YellowTop was charging the PLB40 via its DC input port.

5) Within a three hours, the PLB40 was at 66% and 13.06v, and the YellowTop was 8.39v. I left them connected, but the PLB40 charge never increased beyond 66%.

6) The next time I start the engine, the Genesis solenoid will link the YellowTop back the the alternator and recharge it.

7) The YellowTop has a C20 capacity of 55Ah and the PLB40 is listed at 512Wh of energy, 40Ah.

So what does this all mean? Clearly the YellowTop can charge the PLB40 to a voltage higher than the resting voltage of the YellowTop. But I don't understand why the YellowTop couldn't fully charge the PLB40 if it has a higher C20 capacity rating. Does that mean the YellowTop wasn't fully charged when I connected it to the PLB40? Even with a resting voltage of 12.65v?

 

[jacobconroy]

Clearly the YellowTop can charge the PLB40 to a voltage higher than the resting voltage of the YellowTop.

In your test scenario, if you disconnect the batteries (after the YT has charged the Li) and wait for an hour, then check the Li voltage again I think you will find that it is lower than `13.06 (resting voltages tend to drop a bit once the battery "settles down"). Wouldn't surprise me if both the YT and Li wind up matching at 12.XX V after settling.

I know very little about Li, but I'll bet that if you want it to charge back up to 13.66 you will have to supply it with 14+ volts.

I would try to charge the Li again with the AC adapter and measure the voltage that the AC adapter is putting out. If it's 14+....measure the voltage of your alternator when the rig is running. Hopefully it's 14+ as well. If your alternator has a low voltage output (13.X while running) then you might need a DC to DC charger to "up" that charging voltage to fully charge the Li battery when running.

A resting voltage of 12.6 is fully charged for lead-acid in my experience.

Disclaimer: I'm a novice with all this. Don't take my word for any of it. Testing is the best method. Now that you know that you need to achieve 13.66V on the Li for 100%, the next step is to find a way for your car to get it to that voltage. I don't think charging from the YT is the way to do it.

 

[john61ct]

PLB40 looks like a decent little portable powerpak.

They should have accommodated higher DC charge rates, maybe even 60A with overtemp protection and a warning that going over 15A would reduce longevity.

And a safe jumpstart-output with both overtemp and low-voltage protection would be nice.

Oh and ability to parallel that seems silly to me.

None would increase the cost by much.

The real flaw is not making replacing the internal cells easy.

 

[MattJ]

Eight hours after disconnecting, the PLB40 still reads 13.06v and the YellowTop reads 8.8v.

So the conclusion is that it is fine to set up my configuration as described and pictured in the original post? But just assume that I'll only get 150-200 cycles from the YellowTop instead of 300-400? Or no, I can't do this configuration at all? Or yes, I can do it, but need to make some changes and additions like an A/B switch and adjustable setpoint DC/DC charger cable? Sorry for being slow here, but now that all the data is in, I'm not sure what I specifically need to do.

Thanks!

 

[john61ct]

I'm not sure of your "big picture" goals here, why you have embarked on this specific experiment.

So some feedback, perhaps too specific but might help.

How much was each battery?

Optima will indeed be fast murdered if you make this a habit, not what I call a good deep cycling battery, and should not be often drawn below 50% of its 20-hour Ah rating. 10.5V is a murderously low point, standard 0%SoC for lead capacity load testing, maybe a half dozen times in a lifetime and to be immediately recharged upon touching.

I charged the PLB40 at home with the AC adapter to 100% and measured its voltage at 13.66v.

Excellent, and rare. If you see 13.8V in other circumstances, stop there or a bit lower at low current rates.

I plugged the PLB40 into the Dometic fridge and let it run for four days, checking the voltage a couple times. At 44% the PLB40 was at 12.79v, 35% was 12.69v, and 6% was 10.99v.

Way too long. To get decent lifespan from LFP, stop well before hitting 12V, especially at low current draws. Really 12.4-6 actually higher is better, recharging every day would be best.

At 6% and 10.99v, the fridge had shut down and was blinking an error code (presumably a low-voltage cut-off).

That is not to protect the battery but the fridge, don't let a battery you care about go that low.

An adjustable LVC set to the above cutoffs would be ideal, but otherwise get in the habit of checking yourself, frequently as you get closer to the cutoff point.

I disconnected the PLB40 from the fridge. I measured the resting voltage of the YellowTop at 12.65v. Then I connected the charging port of the PLB40 to the YellowTop using the auxiliary cable that I usually power the fridge with. So the two batteries were not connected in parallel, rather the YellowTop was charging the PLB40 via its DC input port.

Why would you do that?

YellowTop was 8.39v. I left them connected, but the PLB40 charge never increased beyond 66%.

Maybe want to set a record for murdering a lead batt in the shortest number of cycles?

Expensive science project. . .

I don't understand why the YellowTop couldn't fully charge the PLB40 if it has a higher C20 capacity rating.

All about voltage. There likely are voltage conversion circuits internal to the LFP unit.

You really don't want to do this.

If anything recharge lead from LFP, not v/v.

Get some solar or a genny if you're planning to be off-grid more than a day or two.

 

[john61ct]

Have you got a decent ammeter? Ah counting SoC gauge(s) would be ideal. . .

Carefully parse all I've written, google to clarify or ask specific Qs until it starts to get clearer.

 

[MattJ]

Very clear and very helpful. Exactly what I needed. Thanks!

 

[jacobconroy]

Eight hours after disconnecting, the PLB40 still reads 13.06v and the YellowTop reads 8.8v.

Interesting. I would never in a million years have thought that the Li would charge to a higher voltage than the YT has to give. Maybe the lithium has lower resistance? If anyone knows why it worked that way I'd be interested to hear the explanation.

 

[john61ct]

As stated above, built-in DCDC in the powerpack.

 

[jacobconroy]

As stated above, built-in DCDC in the powerpack.

Ah, gotcha. The acronyms make it very hard for newbies to follow along. SoC, LFP, LVC, v/v. (state of charge, lithium something-something, line voltage cutoff, and no idea what v/v means). That's what I think they mean from context....but, see what I'm saying?

 

[john61ct]

price of learning

vice / versa

if google doesn't help, just ask

 

[MattJ]

OK - how about this idea: can I charge my PLB40 battery using the inverter in my trailer, which is connected to dual Group 31 deep-cycle batteries that are charging via the 12v input from my tow vehicle while driving the two vehicle?

 

[john61ct]

You can. While powered by an ICE power source, you may not care that such a redneck-engineering kludge is wildly wasteful.

But once you stop driving, sitting off-grid fed by solar, energy efficiency becomes paramount.