Mixed Battery Sizes/Types On Same Charger?

[richard5933]

I've got a 12v house battery bank made of four Trojan L16 batteries. They are being charged by a Progressive Dynamics 9270 charger which puts out 70 amps. The generator has its own start battery, which is currently an 12-volt 8D Deka start battery. (I know, it's overkill for the generator, but it came with and is nearly new).

The question is this...

The charging circuit in the generator is very old-school and will probably boil the battery if used for too long. There is no alternator on the gennie, and no place to easily install one. Would it be acceptable as an alternative to use an automatic solenoid to bridge the 8D starting battery to the house battery bank? The thought is that whenever the generator is running it will be connected parallel to the house battery bank and receive a charge from the Progressive Dynamics 9270. After the generator was shut down, the solenoid would disconnect the starting battery from the battery bank, and there would be no chance of waking up in the morning with a generator start battery too weak to restart the generator.

My only real concern is adding a battery of a different type and size to the house battery bank while charging. I didn't want to create a situation where either the house or start batteries would over/under charge.

My gut feeling tells me that this is just going to end up being similar to bridging house and chassis batteries together for the purpose of charging off the engine alternator while driving.

Interested in seeing other's thoughts on this. Thanks for your help.

[wolfe10]

Richard,

Couple of questions:

Are both battery banks the same battery technology (i.e. wet cell).  If so, a plus.

A little confused by these sentences-- how/by what means does the generator charge its battery: " The charging circuit in the generator is very old-school and will probably boil the battery if used for too long. There is no alternator on the gennie, and no place to easily install one. " Most RV generators do not have a separate alternator.  Instead, they "power" the converter, charger or inverter charger which charges the batteries. 

If you are going to use the "house bank" converter to charge the generator battery as well, why not use one of the full time smart combiners?

 

We are probably getting into "what is better for both battery banks" rather than "in an ideal world, how would an electrical engineer design it".

 

[dickandlois]

Richard,  There is no alternator on the gennie, and no place to easily install one. 

          Generators installed in coaches do not have alternators as a general rule.

Would it be acceptable as an alternative to use an automatic solenoid to bridge the 8 D starting battery to the house battery bank?  The 8D - is it used as the Chassis engine starter battery  as well as being used to start the generator? This is a common setup so one can start the generator or the coach if the house batteries get discharged. 

The setup used in coaches that have a emergency start switch engage a Large  solenoid that connects the house batteries to the chassis batteries to enable one to start the engine if the chassis batteries are weak.

Rich.

[kaypsmith]

More information on your generator will be very helpful. Old school technology really doesn't give it to us. Many generators do not have an alternator to charge the generator battery, but instead are tapped off of the generator winding and then use bridge rectification with circuitry to give correct charge to the battery. If this is the case, with a good 8D battery, I would not be concerned that the 8D would not start the generator. I was typing while Rich was, so I will not repeat what he has already added. The way to test if the generator is charging the generator battery or not is to check the voltage at the battery with a good VOM, if 13.2 or above just after starting the generator then the genny is charging the battery. Now in an ideal world, I would only add a good (120 volt supplied), battery maintainer to the genny battery in case of a parasitic drain on the battery. And as Rich said if the genny battery gets its supply from the chassis battery bank, then a maintainer would help in helping to keep up the chassis batteries. I really doubt that yours is tied to your chassis battery though, I see in your avatar that your coach is a bus conversion, if so the coach will be a 24 volt system and yes you could tie one 12 volt in, but your coach appears to be one produced before equalizers were added, and this scenario would shorten the life of the coach batteries. Most, but not all generators that have a wattage designation that ends with 500 are built in the way that I described earlier, usually seen with a 25 amp and a 30 amp fuse in the output circuit breaker to your 120 volt output. The windings will be identical, but that is where the other 500 watts is going, to charge the generator battery. Please let us know what generator, both brand name, model#, and date of manufacture if possible, so that we can be more helpful in identifying what is a (best) scenario for you. Almost forgot to mention that Brett's full time smart combiner is also a good choice.

[richard5933]

A little more information:

1. The 24v coach system is totally separate and not what we're talking about here. There are no connections between the coach 24v system and the house 12v system. The generator has its own 12-volt 8D starting battery.

2. All the batteries under discussion are flooded cell.

3. The setup as I got it was what was done in the original conversion in 1974 by Custom Coach. They used the 8D as both the generator start battery AND the house battery. Not a great setup since the battery is not a deep cycle and of only limited usefulness when not running the generator. This is why I'm installing a new house battery bank made of four Trojan 6-volt L16 batteries. They will be set up to run as a 12v house battery bank by being connected series/parallel.

4. The new 120v charger is being connected to the house bank to provide charge whenever we're on shore power or generator. It puts out 70 amps. This is the level of charge recommended by Trojan (approx 10% of battery bank capacity).

5. The generator is a 70s-era Kohler 12.5kw powered by a Perkins diesel. Generator model is 12.5RCOP67. Only 900 hours on the generator, and other than a few minor oil leaks which I'm fixing it runs ok. There is a battery charging circuit in the Kohler, but according to the manual it is an on/off system and will keep charging even when the battery doesn't need a charge. Sounds like a setup to boil a battery, and I won't be using this. Hence the need to come up with some way to charge the generator start battery.

There are two options I see as most viable. The first is to install a solenoid to bridge the 8D generator start battery to the house bank whenever the generator is running. This would charge the 8D along with the house batteries from the 120v charger. My concern here is that the house batteries are deep cycle and the 8D is not, and also that the house batteries and the 8D have different capacities. I didn't know if I'd end up with a situation where one set would be overcharged and one would be undercharged. Or, if this would create a situation in which my house batteries did not ever fully charge.

The second option is to use something like the Xantrex Echo Charger. It would siphon up to 15 amps off the house batteries whenever they are being charged and send it to the generator start battery. My concern here is if 15 amps would be enough to keep the need of the generator for 12v while running and still be able to top off the battery. The generator needs a few amps to run it's electric fuel pump and a few to keep the fuel solenoid open. This is probably my preferred option.

Thanks again for any advice on making this decision.

Edited November 4, 2017 by richard5933
Additional information added.

[kaypsmith]

5 hours ago, richard5933 said:

The second option is to use something like the Xantrex Echo Charger. It would siphon up to 15 amps off the house batteries whenever they are being charged and send it to the generator start battery. My concern here is if 15 amps would be enough to keep the need of the generator for 12v while running and still be able to top off the battery. The generator needs a few amps to run it's electric fuel pump and a few to keep the fuel solenoid open. This is probably my preferred option.

 

The #2 option would be my first option, and yes 15 amps @ 12 volts will be plenty sufficient on an 8D battery for the intended use, few maintainers are in excess of two amps, and the Xantrex Echo charger will monitor both sides simultaneously and perform very well. Also your 8D is not like an auto high cycle battery, nor is it a true deep cycle, and it will actually pair very well with the golf car batteries, but to your original OP, yes the solenoid that you described will also work well for that use, make sure that it is a continuous use solenoid.

[wolfe10]

Agree, option #2.

[dickandlois]

Richard, With all the ducks lined up it makes it much easier to picture the rework!

Option#2 looks like a winner and with Kay's thoughts regarding the  Solenoid type. Things should work  out well.

The generator is a 70's-era Kohler 12.5 KW powered by a Perkins diesel. This Generator most likely has 2 - 30 amp. breakers  powering the main Circuit panel - with an appropriately sized ATS.  

Rich.

[richard5933]

1 hour ago, DickandLois said:

Richard, With all the ducks lined up it makes it much easier to picture the rework!

Option#2 looks like a winner and with Kay's thoughts regarding the  Solenoid type. Things should work  out well.

The generator is a 70's-era Kohler 12.5 KW powered by a Perkins diesel. This Generator most likely has 2 - 30 amp. breakers  powering the main Circuit panel - with an appropriately sized ATS.  

Rich.

Definitely a 70's era unit. I believe that everything in our coach is as it was when it left Custom Coach in 1974. Only 40k miles on the coach, and only 900 hours on the generator. The generator has the two 30-amp breakers and feeds the main panel through a manual transfer switch. I actually prefer the manual switch.

After reading the comments, I am pretty set on just going with the Echo Charger for the 8D battery. At least until I have reason to abandon that method. I can always replace it with a constant duty solenoid if I find that the 15 amps is not enough. I do like the fact that the Echo Charger will not allow the house batteries to ever pull from the 8D so I shouldn't have issues with a dead battery due to something in the house system draining it.

Thanks for all the suggestions.

[kaypsmith]

Richard, I too am running a bus conversion, I recently installed a 5000 (10,000surge watt) watt pure sine wave inverter, it is powered by the two 8D's chassis batteries, and only run while the coach is running. The alternator is 180 amp @24 volts, in turn, I can run the two roof airs off of this setup while driving the bus, has worked very well for me. Forgot to mention that I have also installed a gasoline powered 24 volt generator to give supplemental amps to the battery bank, it an automatic start triggered by 12.0 volts to start the genny and shuts down when system reaches 12.8, it very seldom comes on as I do not use the inverter if the bus motor is not running. The inverter is a 240 volt inverter and can be used to charge the coach batteries if plugged into 50 amp shore power.