Multiple Charging Sources

[White Dog]

After wiring my solar system, I got to thinking about multiple charging systems feeding into one set of batteries. Let's assume for the moment that I am driving down the highway with the camper plugged into the truck and the solar panel on the rood of the camper is putting our near maximum because it's a nice sunny day. Say I've been using the battery a bit and the resting voltage is 12 VDC. The solar controller is in 'bulk' charging mode, so it is at 14.6 VDC. The truck's alternator is limited to 14 VDC and there will be a bit of voltage loss down the wire between the alternator and battery. My question is: Will the voltages from the two charging sources confuse each other into thinking the battery is fully charged or does the battery act like a giant current sink until it reaches a fully charged state? We can leave out all the subtleties of 'bulk', 'absorption', and 'float' at this point. Are the charging sources additive? Do I need to disconnect the solar panel if I want to take advantage of the truck's charging capabilities? This question could be extended to include the converter/charger as well.

Thoughts?

 

[ntsqd]

Simple regulators will be fooled by other charge sources into not charging because of the voltage of the other charge source. More expensive regulators may not be, not sure. I've read some literature that suggests that the more sophisticated regulators can sense actual battery voltage rather than line voltage. Not being a Sparky I haven't a clue how that could be done, but that it can be done isn't a surprise.

If the solar were an unregulated source (typical small panel) then I suspect that it will fool a typical automotive regulator into not charging the camper battery. If it is a regulated source it may still fool it, would depend on the solar regulator's set point vs. the alt's regulator set point.

If you use an ACR/VSR with an external charge indicating light it may have enough power in that circuit to trip a "sugar cube" relay into open circuiting the solar system. A relay logic "Or" gate as it were......

 

[dsrtrat]

My real world experience is that it makes no difference if you have two charging sources. My system works fine with a 65 watt panel and Morningstar basic Sunsaver solar controller. I use the Surepower 1314 battery separator. Both the panel and separator are wired to the camper battery. I have parked for over five days with the Engel fridge running and still had a battery charge so the solar works fine. I watch the amp meter in the truck and can see when it kicks in to charge the camper battery so I know it works in tandem with the solar.

I have never have had a dead battery and the system is going on 5 years with the same 75 amp hour AGM Interstate battery.

I added a switched volt meter so I can check the voltage and try never to go below 12 volts battery charge.

I don't use an inverter as I like to keep things simple and have no need for 120 volt devices in my camper.

I worked on electrical systems for 43 years and one thing I found is simpler is always better.

Dsrtrat


After wiring my solar system, I got to thinking about multiple charging systems feeding into one set of batteries. Let's assume for the moment that I am driving down the highway with the camper plugged into the truck and the solar panel on the rood of the camper is putting our near maximum because it's a nice sunny day. Say I've been using the battery a bit and the resting voltage is 12 VDC. The solar controller is in 'bulk' charging mode, so it is at 14.6 VDC. The truck's alternator is limited to 14 VDC and there will be a bit of voltage loss down the wire between the alternator and battery. My question is: Will the voltages from the two charging sources confuse each other into thinking the battery is fully charged or does the battery act like a giant current sink until it reaches a fully charged state? We can leave out all the subtleties of 'bulk', 'absorption', and 'float' at this point. Are the charging sources additive? Do I need to disconnect the solar panel if I want to take advantage of the truck's charging capabilities? This question could be extended to include the converter/charger as well.

Thoughts?

 

[White Dog]

I worked on electrical systems for 43 years and one thing I found is simpler is always better.



Couldn't agree more. Simpler is almost inevitably better, that is why I started the thread. I've done a bit of digging around since yesterday and only got more confused. Some sources (The Marine Electrical and Electronics Bible by John Payne) is pretty adamant about only using one charging source at a time. Lots of others seem to just blindly hook everything up without much thought to the problem. I think what I'll do is use my DC amp meter and measure the output to a half discharged battery with the solar, then the trucks alternator, and finally with both. I should be able to get some idea of what is going on. Presumably, if the two charging sources don't care about each other, the charge rate with both the solar and truck connected will be the sum (or close to it) of the solar and the truck independently.

On a similar track, I've always been a bit frustrated with slow charge rate from the trucks alternator to the camper batteries, typically 8 to 10 amps. The truck alternator limits output to 14 volts which is midway between 'float' (~13.5 VDC) and 'absorption' (~14.4 VDC)on most smart chargers. The truck's alternator is certainly capable of charging faster but is limited by the regulator. The Australians seem to have figured this out (Redarc) but I can't find an equivalent product in North America. Anyone know of a domestic manufacturer/supplier?

 

[ntsqd]

Look at the Xantrex line of products. I think that you may find something there. If not, investigate blue water marine charging systems and components.

 

[K7MDL]

My take is that for the typical 3 sources - solar charger, AC powered converter/chargers (most factory systems are constant voltage) and the truck charging on our systems, it is of no concern.

There are only 1 or 2 on at a time normally, solar being on always in daytime. The truck is on relatively small percentage of the time. The solar chargers often have maintenance modes or multiple states based on voltage and time rules.

The truck and AC powered factory converter will more or less operate at a single maximum voltage. Each will have its own unique backoff/turnoff voltage setpoint, so one will give up before the other and when both are happy, all charging will diminish or cease. They also put out a lot of current so will blast away until the voltage comes up. They will dominate over the solar most of the time they are on.

The 3 and 4 state chargers, like a solar is likely to be, or in my case the add-on 4 state charger option on my AC converter, can be fooled and mess up it's idea of what it needs to do, but I doubt it is of concern over the long run. In the winter the solar output is often so low that the AC converter output will govern the state of charge.

The one that has the highest tunroff setpoint will rule. The one that has the highest charge current to offer below the lowest setpoint of all charge sources will rule. Other sources will chip in or just watch as they are likely to be fooled into thinking the battery is fully charged seeing a high voltage.

Long term, if the multiple sources were all running a long percentage of time, the desulfurization and 7 day charge modes if you have them will probably be ineffective or less effective.

I only use the AC in the winter when solar is useless and the truck is off. When the truck is on the AC is off. So the only time I really have 2 sources charging is summer with the truck running, and that is not very long to be of concern. See, it is simple .

 

[White Dog]

Look at the Xantrex line of products. I think that you may find something there. If not, investigate blue water marine charging systems and components.

Xantrex and Blue Sea are my usual first go-to checks for electrical components but, in this case they let me down

 

[White Dog]

My take is that for the typical 3 sources - solar charger, AC powered converter/chargers (most factory systems are constant voltage) and the truck charging on our systems, it is of no concern.

There are only 1 or 2 on at a time normally, solar being on always in daytime. The truck is on relatively small percentage of the time. The solar chargers often have maintenance modes or multiple states based on voltage and time rules.

The truck and AC powered factory converter will more or less operate at a single maximum voltage. Each will have its own unique backoff/turnoff voltage setpoint, so one will give up before the other and when both are happy, all charging will diminish or cease. They also put out a lot of current so will blast away until the voltage comes up. They will dominate over the solar most of the time they are on.

The 3 and 4 state chargers, like a solar is likely to be, or in my case the add-on 4 state charger option on my AC converter, can be fooled and mess up it's idea of what it needs to do, but I doubt it is of concern over the long run. In the winter the solar output is often so low that the AC converter output will govern the state of charge.

The one that has the highest tunroff setpoint will rule. The one that has the highest charge current to offer below the lowest setpoint of all charge sources will rule. Other sources will chip in or just watch as they are likely to be fooled into thinking the battery is fully charged seeing a high voltage.

Long term, if the multiple sources were all running a long percentage of time, the desulfurization and 7 day charge modes if you have them will probably be ineffective or less effective.

I only use the AC in the winter when solar is useless and the truck is off. When the truck is on the AC is off. So the only time I really have 2 sources charging is summer with the truck running, and that is not very long to be of concern. See, it is simple .


My suspicions are they same as yours. However, this does create a bit of a problem. If I've been stationary for 2 or 3 days and the battery is low, when I hit the road, the truck's alternator is only going to put 14 VDC. That's its cutoff voltage. The solar panel will be putting out 14.5, tricking the truck into thinking the battery is charged. So driving all day is going to result in no charge from the truck, just the 5.5 Ah (maximum!) from the solar panel. At the end of the day, my batteries will have very little charge compared to what the truck would have contributed if it didn't think the battery was fully charged.

 

[K7MDL]

My suspicions are they same as yours. However, this does create a bit of a problem. If I've been stationary for 2 or 3 days and the battery is low, when I hit the road, the truck's alternator is only going to put 14 VDC. That's its cutoff voltage. The solar panel will be putting out 14.5, tricking the truck into thinking the battery is charged. So driving all day is going to result in no charge from the truck, just the 5.5 Ah (maximum!) from the solar panel. At the end of the day, my batteries will have very little charge compared to what the truck would have contributed if it didn't think the battery was fully charged.



The only way the solar will get to 14.5 is that the battery is charged enough to rise that far. The truck with greater capacity (I assume) should contribute the lion's share of the charge until the voltage rises enough to level off, and then the solar will take it the rest of the way. By that time it is pretty well charged. What you have here is the battery voltage rises only as far the charger voltage and capacity can pull it (until it reaches near full charge - after that raising the charge voltage will cause overcharging and gassing).

The more discharged the battery, the harder it is to raise the battery voltage since the charger is likely maxed out compared to what the battery can take (initially). As the battery charges, the voltage rises and the current rate falls off, then it makes no difference who is supplying the current, not much is needed.

I would further add that your truck is monitoring the voltage and temperature close to your truck battery and it will be the combined voltage/charge state that controls the output. Until the camper battery is near fully charged, the voltage drop of the camper battery will allow the truck charger to think it is charged sooner than normal since the voltage at the truck battery will be higher than your camper battery. Large shaorter cables to the camper help minimize this, as does choosing similar battery types, or using a flooded cell camper battery (vs AGM) that uses a slightly lower charge voltage. That option is not likely for most of us. I would think you have nothing to worry about, just use it, it should work just fine though the last bit of charge will be limited by the solar max rate. The only way around this is to use a dual charger setup, which would need the ability to boost the voltage at the camper battery (expensive), or relocate the truck charger temp and voltage sense to the camper (not likely).

 

[K7MDL]

I am curious, how did you determine the truck puts out only 14.0VDC? Was this at the camper battery? With a full or discharged battery? What size wire are you running from the truck battery to the camper? Same size all the way? Connector types? Is it clean and tight? Using a diode isolator or a voltage sensing relay?

These all have voltage drops associated with them that need to be taken into account and perhaps have room for improvement.

 

[White Dog]

I am curious, how did you determine the truck puts out only 14.0VDC? Was this at the camper battery? With a full or discharged battery? What size wire are you running from the truck battery to the camper? Same size all the way? Connector types? Is it clean and tight? Using a diode isolator or a voltage sensing relay?

These all have voltage drops associated with them that need to be taken into account and perhaps have room for improvement.


I've measured the voltage at the truck battery with the engine running. It's 14.0. I haven't measured it at the camper battery yet but I will the next time the two are together. In my discussion above, I was assuming there was minimal voltage drop from the truck battery where the cable to the camper is attached to the camper batteries. This is, of course, an incorrect assumption. It looks like FWC Canada used #10 wire for the run from the truck battery to the plug in the truck box (about 10'). From the AWG cable voltage drop tables, I am guessing the voltage drop is about 3% or 4% (around .5 volt). I will actually do the measurements the next time the camper is on the truck.

I think you have answered my question though. If I understand you correctly, the battery will act like a current sink for all the chargers until it reaches the cutoff voltage for each. If the truck and solar panels are both charging, the truck will likely stop first since it has the lower cutoff voltage. The solar panel will continue on until the battery is fully charged - the solar controller being 'smarter' than the truck with respect to the needs of the camper battery.

Back to the truck for a second. Voltage drop from the truck alternator to the camper battery would seem to be quite important. Therefore, would it make more sense increase the gauge of the wire from the truck battery to the camper (within reason) minimizing the voltage drop? This would give a bit of an improvement with minimal cost.

 

[K7MDL]

...

Back to the truck for a second. Voltage drop from the truck alternator to the camper battery would seem to be quite important. Therefore, would it make more sense increase the gauge of the wire from the truck battery to the camper (within reason) minimizing the voltage drop? This would give a bit of an improvement with minimal cost.


Given you have solar and assuming it is effective when you need it (daytime and when AC power is not available) it may not be that critical other than not getting your battery recharged to the fullest at the fastest rate. #10 is OK and as the battery approaches full charge and the current drops, so does the voltage drop. I happen to run #6 from the battery to the truck bed connector, after that it is stock #10 inside the camper. Your choice of truck bed connector needs to be able to handle any larger sized wire also. If it was easy to swap out, I might choose to upgrade that section of wire. Else, personally I would leave it alone and let the solar top things off.

 

[ntsqd]

I've been researching solar since our rig currently lacks it. What I have noticed is that the better solar controllers are actually DC-DC converters (if I understood their operation correctly). Assuming that this is correct they don't actually regulate the solar panel, they simply convert it's output into the voltage and current form that the battery's state of charge needs.

Given that - and this just occurred to me, why not buy a solar charge controller with enough ampacity to handle the alternator's output and the panel's output combined? Wire both to the controller's input. Then you have two charge sources, but only one (fairly sophisticated) unit controlling the camper battery's charging. Assuming that this would work you could possibly also connect the AC charger's output to a large enough solar charge controller. Give the charge controller sole control over how the battery is charged, regardless of charge source.

FWIW I too went with 6AWG for both charge cables. They are currently hard-wired to the camper (time constraints) but will shortly be disconnectable via an Anderson charge plug from Grainger.

 

[MarkBC]

What I have noticed is that the better solar controllers are actually DC-DC converters (if I understood their operation correctly).

MPPT (maximum power point tracking) charge controllers -- that's what I have. From what I've read (I'm not an expert) they're superior because the solar panel will frequently/usually be putting out a higher voltage than your battery can use (like, 16 - 18 volts), and that extra power (volts x amps) is wasted.
The MPPT senses what voltage the battery needs and what voltage the panel is putting out and converts the DC voltage from the panel to high-frequency AC then back again to whatever lower-voltage DC is appropriate for the battery's needs to feed it the maximum current. I've read that the efficiency of conversion (with modern digital MPPT) is ~95%, so as long as the MPPT is saving more than 5% wasted power then it's worthwhile.

If camping in cold (but sunny) weather, MPPT is even more-better. PV panels put out higher voltage when cold than when hot, so the match between panel-output-voltage and battery-required-voltage is even worse when it's cold out, so MPPT becomes more useful.

 

[White Dog]

Given that - and this just occurred to me, why not buy a solar charge controller with enough ampacity to handle the alternator's output and the panel's output combined? Wire both to the controller's input. Then you have two charge sources, but only one (fairly sophisticated) unit controlling the camper battery's charging. Assuming that this would work you could possibly also connect the AC charger's output to a large enough solar charge controller. Give the charge controller sole control over how the battery is charged, regardless of charge source.



A good idea in theory but I think the flaw would be that the truck's alternator is putting out less voltage than required whereas the solar panel is putting out more. I'm no expert (obviously) but I would guess one cannot do the other's job. As I mentioned earlier, the Australians make step-up controllers (Redarc ) they call 'DC Battery Chargers' but I can't find any domestic sources for similar items. Redarc doesn't seem to export to North America so they wouldn't be UL or CSA approved (bad for my insurance). Besides, they are kind of pricey ($671 AUD).

 

[ntsqd]

A variable DC to DC converter can take in whatever voltage and current is available, and convert it to a suitable voltage (up or down from the input) along with the attendant current adjustment. Whether the solar controllers are that flexible is something that I don't know.

This http://www.windsun.com/ChargeControls/MPPT.htm would seem to indicate not, but I couldn't make the claim that All are like that based on one net page.

If the truck's alt is going to function correctly for charging the truck's battery(ies) then it should also be functional for charging the camper battery(ies). If it isn't delivering a high enough voltage for proper charging to the camper battery(ies) then that is not the fault of the alternator, and it should be corrected first.

 

[pods8]

If you want a DC-DC charger that can boost up voltage to stage charging there are only 2 that I know if when I was looking. Powerstream has one, which I have in my FWC, but it's limited to 7.5amps. (On my new build I'm just going to run large gauge wiring to limit drop and let the solar do the conditioning/overcharge voltage.) Also promariner makes a 40amp one, but it runs about $400 I believe. Neither of these take solar input as well which seems like the redac you were linking to does.

 

[pods8]

Given that - and this just occurred to me, why not buy a solar charge controller with enough ampacity to handle the alternator's output and the panel's output combined?


I spoke with morningstar back in the day to inquire if I could do that with their MPPT products and they said it wasn't designed for it and wouldn't work. Just one data point.

 

[UglyScout]

At what point are we spending a ton of money to get either a few more amps out of our battery per cycle OR a few more cycles out of the battery over its lifetime?

 

[craig333]

My truck alternator is rated at 125 amps. That'd take a pretty big controller. I can switch mine from the truck to the solar or both together. I'll have to try and take some measurements this week and see if I can see anything.