A constant voltage converter and a smart charger together on the same battery circuit

[Lance990]

My question involves using a smart charger in combination with a re-purposed Magnatek 7345RU converter/charger that still works and puts out a steady, regulated 13.8 volts DC. I have the 7345RU from when I upgraded my truck camper converter to a PD 4645 a few years ago. The Magnatek will charge a regular lead acid battery, given enough time, but it won't ever charge the AGM battery up to 14.4V where it needs to bulk charge. The converter works great and can supply up to 45A DC with no sag, which is way overkill for my needs but I had it in my garage from when I upgraded the converter in my truck camper to a 3-stage smart charger.

Here's the why I am asking:

I have a 12V fridge in the bed of my truck when I am not using the truck camper (the bed has a topper) and by using the 7345RU, I can pull into my garage and plug in the converter which will supply power to the fridge without the truck running. I am using a VMax 135AH AGM battery to power the fridge that is isolated from the truck battery unless the truck is running (using a 40A relay). Unfortunately, the converter will not charge the AGM battery fully since it is a single stage converter/charger. I do have an 8-stage 15A smart charger I bought from Vmax that is ideal for charging the AGM battery. Can I use the smart charger to charge the battery while the converter is powered up and supplying power to run the fridge? The smart charger is not designed to be a converter so it cannot power the fridge and charge the battery, concurrently. Would it be safe to have them both connected to the battery at the same time? My theory is that neither will interfere with the other but I am not sure. My understanding is that a solar charger and a converter can easily work together on the same battery without any problems (I have a 100W panel on the roof and a 20A MPPT solar charger). I COULD disconnect the battery and charge it separately using the smart charger because the converter will supply 12V to the fridge without needing a battery but I am lazy and want to just plug in the converter when I pull into my garage. Is this dangerous to do? What are the risks?

 

[john61ct]

Volts are volts amps are amps.

A steady 13.8 is like Float, good for carrying loads and keeping a Full bank at Full.

OK to add the higher voltage input, but does it raise the overall circuit voltage high enough?

Measure V at the battery posts over a 6-8 hour period, ideally also amps acceptance rate.

Full is when that falls to the batt's endAmps spec, often 0.005C

You do want to hold the CV setpoint tat long, but not too long, especially with a sealed type, GEL or AGM.

 

[Lance990]

Volts are volts amps are amps.

A steady 13.8 is like Float, good for carrying loads and keeping a Full bank at Full.

OK to add the higher voltage input, but does it raise the overall circuit voltage high enough?

Measure V at the battery posts over a 6-8 hour period, ideally also amps acceptance rate.

Full is when that falls to the batt's endAmps spec, often 0.005C

You do want to hold the CV setpoint tat long, but not too long, especially with a sealed type, GEL or AGM.

I agree with 13.8V being a good maintainer voltage. This is the charger: https://www.vmaxtanks.com/BC8S1215A-12V-15A-8-Stage-Smart-Battery-Charger-Maintainer-_p_218.html
I am going to mount the charger in the truck bed so that when I plug the truck in that the charger and converter will both power up. I will watch the voltage at the battery terminals to make sure it rises to 14.4V so I can confirm that the charger is working correctly. I am mostly concerned that the smart charger will "see" the converter and will cause problems with the automatic charging profile. I had also considered installing a relay that would take the battery out of the circuit once the converter is powered up so that the load on the battery goes away while it is charging. However, I want to test if I can just use the smart charger "as-is" in the circuit. The refrigerator uses about 45W when the compressor is running and only about 11Wh total so it does not have a huge impact on the battery.

The recommended float voltage for the battery I am using is between 13.5-13.8V so the converter works great for that. This is the battery: https://www.vmaxtanks.com/XTR31-135-12Volts-135AH-Deep-Cycle-XTREME-AGM-Battery-_p_176.html

The 13.8V is on the high end of the float voltage range but I should be okay with that voltage if the manufacturer specs it that way, right?

 

[john61ct]

Yes.

Again, volts are volts amps are amps.

Even the "smartest" charger does not know or care about other sources, if the circuit is already at 13.8V or below 12V, its job is to eventually get it to the CV setpoint.

Just make sure to measure at the bank posts, to adjust for any voltage drop.

 

[dreadlocks]

Yeah in this case one source is sitting at float voltage, if the other is absorbing at a higher voltage that lower voltage source is not really doing diddly.. so even if your 'smart' charger was counting amps to decide when to stop absorbing and switch to float, it would not be influenced by your converter.. but the only chargers that do that need to know the battery capacity or are sized to a specific battery capacity, so its likely doing a fixed absorb like most and even if you had multiple chargers absorbing the'd not interfere in any way.

When you unplug your smart charger, or your smarter charger goes to float that is equal or less than that of your converter.. the battery will quickly come to rest at the 13.8 and the converter will start taking over loads and maintaining charge.

this is fine, just do some testing and make sure its doing what you think it does..

 

[LandCruiserPhil]

Check out Victron one of the only chargers I found that can be used as a stand alone power supply or do both. Another feature is when your battery/ies are 100% full it does keep float going.

 

[Lance990]

Thanks everyone! This has confirmed what I suspected as true and I appreciate you guys tasking the time to respond. My truck camper has 400W of solar into a 30A PWM solar charger and a 40A DC-DC charger between the alternator and the secondary battery and it all works together fine, so it makes complete sense that it would all work with a converter. The solar works fine with a converter and this should be no exception. I just really needed a reality check on my own thoughts.

 

[DaveInDenver]

Even the "smartest" charger does not know or care about other sources, if the circuit is already at 13.8V or below 12V, its job is to eventually get it to the CV setpoint.

That assumes one supply does not upset the other. You'll have two control loops chasing each other if they're both powered. It's usually OK to leave a powered off supply connected since most should be (but not always are) high impedance. But you'd wise to check with the data sheet or directly with the manufacturer to make sure it's acceptable to run both supplies in parallel and even then it's not unusual to have one master and one slave to current share properly or so there's a way to handle faults.

 

[john61ct]

I am saying that is not actually a concern in real life.

There is no "upset", and even if there are multiple disparate charge regulation circuits in parallel, in practice over hundreds of installs, solar / wind / hydro, alternator, PSU/rectifiers, DCDC converters as well as "normal chargers" off both gensets and mains,

never came across a reason to isolate / switch one source off, just because the others are running concurrently.

They can even have widely differing charging profiles, no problem.

Contacting the manufacturer they will likely say only one at a time out of an abundance of (IMO) unnecessary caution.

Of course, there is no harm in doing so either if you want, other than a longer charge cycle in some circumstances.

In this case there is only one "smart" charge regulator anyway.

 

[Lance990]

Well, I gave it a test just a hour or so ago. I plugged in the smart battery charger and turned it on. It went through its checks and landed in the absorption stage at 14.1V where it is programmed to stay until current drops to 2.25A. It then went to float at 13.77V. The battery was apparently almost fully charged from the solar controller before I pulled in the garage so I won't likely need the 120V smart charger very often except when the battery is in a SOC less than 90%. Here are some photos of my setup:

The truck:



My homemade powerbox and converter:



The external male 115V plug:





The rest of the bed:



I made a tailgate/wall for using an 5000 BTU air conditioner:



Me enjoying the AC:

 

[john61ct]

Yes real life testing requires

first turning off all charge sources

run loads until bank at 40-60%

then start charging

so that you can observe the V&A curve over the 6-8 hours required to get back to Full.

 

[Lance990]

Yes real life testing requires

first turning off all charge sources

run loads until bank at 40-60%

then start charging

so that you can observe the V&A curve over the 6-8 hours required to get back to Full.

I will do that this weekend when I have more time. I checked the output amps of the smart charger and it was pulsing at around 4.89A in the absorption stage but did not get enough data to measure the curve. The combination of low draw, alternator charging and solar charging mean the battery is mostly in a float service application. I don't need much power because this setup is mainly for weekend trips and overnight camping. I'll use the larger, more comfy Lance truck camper on longer trips. However, I am liking the lower weight of this setup compared to the truck camper (3,800 lbs) and it is more suitable for off-road traveling. The height of the camper and the 3 foot overhang are very limiting. This is my "mini RV" and is far more efficient and easier on gas with the big V10 under the hood. My wife, however, loves the shower and toilet in the Lance. She did camp with me the other night in the truck bed at a campground where I set up the air conditioner and ran it all night. She said it was a very pleasant experience and she even used the Luggable Loo! Baby steps...

Having this setup makes everything more flexible. On the weekends, we can make excursions to get out of the city and away from people and still have everything we need to be comfortable. I can see myself traveling across the country with just this setup and occasionally stopping at a campground and plugging in on hot nights to run the AC. I am an old tent camper so this is definitely more comfortable than sleeping on the ground.

 

[Steve_P]

My question would be is the converter even needed. My fridge, if plugged into both 110 AC and 12 DC will select the 110 AC for power. In which case I would only need a dual110 outlet, essentially a dual outlet extension cord, to power both the fridge and the battery charger.

 

[Lance990]

My question would be is the converter even needed. My fridge, if plugged into both 110 AC and 12 DC will select the 110 AC for power. In which case I would only need a dual110 outlet, essentially a dual outlet extension cord, to power both the fridge and the battery charger.

The fridge in my Lance truck camper is similar (3-way) but the Ausranvik 26 qt compressor fridge I bought off of Amazon has 2 power cords. One for AC and one for DC and only one of them can be plugged into the unit at a time. I got tired of switching the cords when I pulled into the garage so I decided to just keep it on DC. Besides, the AC cord has a power brick on it so it was doing the AC to DC conversion, anyway. Now, I just plug one cord into the truck and the fridge keeps working while the battery is getting a charge. If the battery is at 100% (which it usually is), I don't even need the smart charger. But, I will take the smart charger with me om trips and if I DO have a low battery situation and the solar can't keep up (100W subject to cloudy conditions), I can use the smart charger on the same circuit as the converter and all will work together peacefully to complete the battery charge. I also use a 5000 BTU air conditioner, as needed, when sleeping so I can camp at any campground and just plug in one cord and everything works. It's sorta like a "mini RV" in the bed of my truck.

What is the make and model of your fridge?

 

[Steve_P]

Now I'm running a Dometic CFX 40. Before that I had a CF 25, which only ran on 12 volt. So when shore power was available I would put a charger on the battery, which was able to fully charge the FLA battery while keeping the fridge running, so I understand what you're doing.

I would recommend that if you're planning to run an A/C, the converter and the battery charger simultaneously you consider using a detachable 10 gauge 30 amp RV service with at least 2 15 amp breakers. 15/30 amp adapters aren't that expensive and you can use one at home when you're only running the converter and charger.