Having a hard time keeping my Dometic PLB40 battery charged (1 Viewer)

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Would a weak starting battery affect my ability to charge my Dometic PLB40 house battery?

I’ve had this PLB 40 for about six months and it generally works pretty well. But lately I’ve noticed that my vehicle, LX 470, has a hard time keeping it charged. I have it plugged in to the 12 V accessory outlet in the rear. That outlet only works when the vehicle is running so I assumed the starting battery has nothing to do with it. But my knowledge of auto electrical systems is clearly lacking. Is the starting battery involved in charging the house battery even if it’s not wired hot to it?

The reason I ask is that yesterday during an oil change the guy checked my starting battery and pointed out it was getting a little weak. It still has plenty of cranking power and I didn’t think much of it until I noticed the PLB40 not charging well again.

When I plug it into that rear 12 V outlet, it will charge for a few seconds and then the arrow indicator disappears and it starts to slowly lose its charge. It’s not a blown fuse because if I unplug it and plug it right back in it will charge again for a few seconds. I’ve tried plugging the battery into the other 12 V outlets in the front of the vehicle with the same results. This morning I started with the PLB40 battery at 100% (charged via AC at home). I drove about 30 minutes and it was down to 98% while being plugged in the entire time. It’s running a Dometic CFX35 fridge, but that fridge was pre-cooled (also via AC at home) and loaded with ice cold pre-cooled drinks. Demand from that fridge should not be more than the vehicles ability to charge the battery. But it is.

I run a Renogy 100w Solar panel on the roof as well but I don’t use it often because I park in the garage and anytime the vehicle is out in the sun it’s usually when I’m driving, so I use the 12v outlet instead. I’m headed out on a long road trip today and as soon as I get to some sunnier weather I’ll see if the solar panel does a better job of keeping up with the demand.

anyway, do you think me getting a new starting battery will help solve this problem?

thanks in advance.
 

e9999

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the 2 batteries are wired together via the cables to the outlet, so they should basically have the same voltage minus or plus the voltage drops in the wiring due to currents when actually connected. If your starting battery is at a low voltage, say, the house battery will then either stay low too or if charged fully beforehand, as you just did, will charge the starting battery, therefore discharging itself. It's also possible that your house battery charger has a disconnect system that prevents it from charging if the source (the starting battery) is too low, preventing house battery charging. And there could be odd things going on in the battery control system since LiFeP normally should be charged at higher voltages than a FLA. Throw in unknown behavior of your alternator and it's not very meaningful to hazard guesses. Of course, if the outlet in the back is controlled by the key, then all this is only happening with the key on.

So I would check the condition of the starting battery by monitoring its voltage at rest and when being charged. Easy to do, get a multimeter if you don't have one yet. You need one around in any case. Next, if there are indications of problems, like low voltage or rapid loss of voltage after charging, do a battery condition test. You can buy a tool to do this, or have it tested at a parts house or shop, or improvise a discharge test yourself with some bulbs or an inverter. But how old is that starting battery? If it's more than, say, 3 or 4 years it's probably getting weak. 5 or more and it's quite likely it's on its last legs. In that case, it's probably a safe bet to buy a new starting battery anyway for peace of mind.

If the battery is good or even at the start of the troubleshooting actually, I'd also look at the alternator. It should keep the voltage up at both batteries. In fact, it seems odd that the alternator would not be able to keep up, even with a weak battery. Even if the starting battery is bad, I'd expect the alternator to keep the voltage at the battery terminals well above 12.6V when the engine is running. Try first without the house battery on. Check the starting battery voltage. Then add the house one without the fridge. Then with the fridge. If the voltage is low, there may be something off with the alternator. You could also put an AC battery charger on and see what happens and if things are different. Either way, best to get a voltmeter and start measuring.
 
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I appreciate your answer, was just heading out for a road trip to Moab when I posted it and just now checking back. Not sure about the age of the starting battery, I just bought the vehicle a few months ago. I put 2000 miles on it this week. Plugged into the 12V it indicates charging but the fridge draws faster than the alternator can charge it. Plugged into the solar panel it gets it right back up to 100% fairly quickly and keeps it there all day. The alternator is almost brand new. I had it done by a reputable shop from this board.

I'llrun the voltage checks you suggested. I have a new group 31 battery I bought a couple months ago for a battery box before I decided to switch to the Dometic PLB. I'm going to see if I can fit it in and use it as a new starting battery if the current one is weak.
 

e9999

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well, if your 100W panel can easily keep up with the fridge but the alternator can't when they are wired the same way to the batteries (so no weird controller or wiring discrepancies involved), that would then suggest that there is something wrong with the alternator (or its specs are not compatible) because it surely should be able to produce much more power than 100W if working normally.
If you want to buy a multimeter and don't need to keep the cost rock bottom, maybe think about getting one that can measure DC current with a clamp. That may come in handy for this and other work later on.
Most batteries have a code stamped or printed on that can tell you the manufacturing date.
You might want to make sure that 31 battery is starting or dual purpose rather than strictly deep cycle if you're going to use it for starting the engine.
 
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Updating this... it was the accessory outlet.

I tested the battery and the alternator with a multimeter. Both showed good voltage. Testing the wires on the back of the 12v accessory outlet in the cargo area also showed a good 14v making it back there when the engine was running. I started to think I had a bad power cable. Its a cigarette lighter plug on one end and an Anderson on the end that goes into the PLB40. But the connections on it seemed solid. I wedged the multimeter probes up in the Anderson plug and got 14v again but I noticed I had to jiggle the other end at the outlet to get a good charge. Swapped out the outlet for a cheap one from the local auto parts store and it's ugly as all get out but... it works. Battery charges like a champ again even when powering the fridge DC to DC.

I'm going to pull the rear panel soon and see if there's room to mount a nice Blue Sea 4366 with a cigarette outlet and a couple USBs and a voltmeter. I've seen them in 200s and also in ARB drawers off to the side, but not sure it'll fit in the interior rear panel or not. That would look a lot nicer than the wires I have coming out of the panel to the new outlet. But for now it's nice to have power again.

Thanks for your response... I did learn a little bit in this whole process. Glad I didn't go buy a new starter battery too.
 

e9999

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Good that it was an easy and cheap fix and that you figured it out fast, sometimes if an electrical problem is intermittent it's pretty hard to diagnose it.
I have installed a marine cig lighter outlet in the back of my 80 but if I were to do it today I'd use an Anderson PP outlet instead, I think that is a better way to go. A voltmeter at the outlet is nice because it will show you easily how much voltage you lose over the wiring when a device is running, that loss can lead to bad surprises if the device shuts off because of insufficient voltage, and especially so for a fridge.
 
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I was thinking sabout going the anderson route. That battery stays plugged in 24/7 and with cargo shifting around the cigarette plug can take a beating.
 

e9999

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well, the Anderson PPs are not mechanically locked per se but there is a fair bit of a resistance to pull out due to the curved contacts sliding/snapping together. From memory I am thinking it is more resistance than your typical cig lighter plug, and I have also seen some cig lighter plugs (presumably with low quality or smushed side contacts) where the plug was pretty easy to remove. And even some where the cig lighter would work itself out because of an overly strong spring behind the central contact. So, yes, I think the Andersons are more secure. Also way more compact. I use them a lot for DC projects.
 

JLH

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I like Andersons but coming apart can be an issue. There's a locking piece available but I just use a 12 ga solid copper bent into a U and insert it into the two holes on a mated set of connectors. If it's bent out a bit it wedges in tight enough to not fall out. Hasn't failed me yet.
 

e9999

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^ yes, easy to improvise. And you just reminded me that I have some of those little plastic pieces someplace, forgot about them. TBF that's because I never had an Anderson PP connector come apart unintentionally so far and I have dozens of them around. (To be completely honest, though, this is not counting the one or 2 times when my feet got tangled in the wire and I was actually glad it opened up instead of me doing a faceplant or a solar controller being pulled off something and taking a dive.) It is possible of course that the springiness of the contacts decreases with time and that older connectors are more prone to unintentional separation but I have not seen that yet.
 

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