I want to trickle charge a 12V SLA battery on a trailer from the truck battery. I have a SunGuard solar charge controller from Morningstar and was wondering if this would work.

Rated solar input is 4.5A. The trailer wiring I have can handle this current. Would it be easier to try to regulate this current and use the controller I have or does anyone have a recommendation for another product?

 

The size of the SLA is needed to decide what a trickle charge is. From that can be decided what is "easier". More details on the Sunguard device would also help.

 

The trailer battery is 36 Ah.

Here is a link to the controller datasheet:
http://www.morningstarcorp.com/en/support/library/SG.DAT.Data_Sheet.01.ENG.pdf

 

I fear that the Sunguard is depending on the solar panels to never excede 4.5 amps of available current. If this is true, hooking this up to a truck battery would smoke it instantly. You can try it, but be sure to use a 4 to 5 amp fast blow fuse to protect the Sunguard, and even that might not save it. Perhaps ask the Sunguard corporation?

The next problem is that a 12 volt battery can not charge a healthy 12 volt battery because you must have a higher voltage to force current backwards through the SLA. If your truck battery is not connected to a truck, you will not have the proper voltage available.

 

I fear that the Sunguard is depending on the solar panels to never excede 4.5 amps of available current. If this is true, hooking this up to a truck battery would smoke it instantly.

This is what I was afraid of. Although max current is 5.5A for 5 minutes...

You can try it, but be sure to use a 4 to 5 amp fast blow fuse to protect the Sunguard, and even that might not save it. Perhaps ask the Sunguard corporation?

This is the possible "easier" attempt to regulate I was pondering. I think I will test this idea. Maybe a PTC like Bourns MF-RHT200 would also work. Will also talk to Sunguard as you suggest.

The next problem is that a 12 volt battery can not charge a healthy 12 volt battery because you must have a higher voltage to force current backwards through the SLA. If your truck battery is not connected to a truck, you will not have the proper voltage available.

I should have been more clear. I only expect the trailer battery to charge when the alternator is running on the truck. The controller will disconnect the batteries if the truck battery voltage falls below 12.7V.

I could also use a resistor to limit the typical charge current. A .5 ohm resistor would limit the current to (13.7V-12.6V)/.5 = 2.2A.

 

I am thinking, why trickle it? A sealed lead acid battery is the same chemistry as a truck battery. Why not just let it charge in parallel?

I would also consider the wiring as a possible source of half an ohm. Do some measurements. Are you that good with a meter?

Attach a light bulb back where the battery will be. Measure the amps, then the voltage drop caused by the wiring, and calculate the resistance.

Then there is the question of running the SLA down, then starting the truck. That can cause some large amps to try to charge the SLA. Do some measurements. Do some math. Consider how you intend to use the SLA. Try running it down to 12.0 volts and measure the charging current. (Use a fuse at the highest charge current you decide to allow.)

You might be over-complicating this.

 

SLA's generally require a bit higher terminal charge voltage than automotive batteries; an exception would be Optima or AGM batteries.

Do you have a link to the datasheet for your particular SLA battery? If not, please find it, or at least post the manufacturer and model of the battery that you have. Believe it or not, SLA charge requirements can vary considerably even with models from the same manufacturer.

Just some rough numbers; you are generally better off to use an 8-hour charge rate for SLA's, as that reduces internal heating. However, some manufacturers say you can charge them at a 5-hour rate.

An 8-hour charge rate would be a current flow of 6A. (36AH/ 75% charge efficiency / 8 hours)
A 5-hour charge rate would be a current flow of 9.6A. Battery heating would be considerably more than at the 8-hour rate.

However, if the SLA is an AGM (absorbent glass mat) type, you can charge it a great deal faster.

 

I am thinking, why trickle it? A sealed lead acid battery is the same chemistry as a truck battery. Why not just let it charge in parallel?

Using the controller would prevent truck battery drain to the trailer battery if the truck battery voltage falls below 12.7 volts. Then I shouldn't have to worry about dead truck batteries.

I would also consider the wiring as a possible source of half an ohm. Do some measurements. Are you that good with a meter?

Good idea. I'll make some measurements.

Then there is the question of running the SLA down, then starting the truck. That can cause some large amps to try to charge the SLA. Do some measurements. Do some math. Consider how you intend to use the SLA. Try running it down to 12.0 volts and measure the charging current. (Use a fuse at the highest charge current you decide to allow.)

Yes, I definitely want to protect against larger voltage differentials (2+ volts) between the truck and trailer batteries. I'll get the PTC I mentioned above plus a fast acting fuse for limiting the maximum allowable current as you suggest.

 

SLA's generally require a bit higher terminal charge voltage than automotive batteries; an exception would be Optima or AGM batteries.

Do you have a link to the datasheet for your particular SLA battery? If not, please find it, or at least post the manufacturer and model of the battery that you have. Believe it or not, SLA charge requirements can vary considerably even with models from the same manufacturer.

It's an AGM battery. I'll get the info on the battery.

However, if the SLA is an AGM (absorbent glass mat) type, you can charge it a great deal faster.

Even though the battery may take a higher charge current, I want to limit the typical charge current in order to protect the Sunguard controller I want to use. I had planned on starting with a 2 amp typical charge current, but 2 amps is only 1/3 of the 8 hour charge rate you listed above. Maybe 2 amps will not be enough to keep the battery topped off. I don't plan to run the trailer battery down very much though...I will have to see how it works out.

 

What you need is a solenoid to connect/disconnect the aux battery from the main battery. Here is a link to the one I've got: http://www.nationalluna.com/intelsol.htm. The solenoid allows both batteries to be charged in parallel while the engine is running. Then only use the solar panel to charge the battery when the trailer is stationary.

A word of warning though, normal trailer wiring will not be sufficient for this purpose, you need at least 16mm^2 cable, similar to welding cable. The battery cable will go though a separate connector, like the third one from the top, on this page (Brad Harrison connector): http://www.nationalluna.com/plugs2.htm

This setup is very popular here in South Africa, where allot of offroad vehicles and trailers are equipped with fridges, freezer and ligthing systems for all sorts of outdoors activities, so it is tried and tested. I've been running this setup for over 5years on my vehicle.