As part of my electrical upgrades, I want to automatically float charge my start batteries when hooked to shore power. This has been mentioned a few times in other posts with no definitive solutions mentioned. To come up with a solution, it is necessary to investigate the "rules" for battery tender operation on a coach application. To this end, I called Schumacher Electric tech support. I like their products and currently own their SP3 model with 3 amp capacity and digital readout (Amazon-$40).

A discussion ended with this information:

- charger on, there is DC output to battery with leads connected.

- charger off and DC leads connected to battery, there is no charge to the battery and no battery discharge back through the charger. The reason for this is that with power off, the charger is automatically disconnected at the it's output. With this situation, the engine can be started and there will be no harm to the charger.

- charger on and DC leads connected to the battery and the engine is started, the charger will burn out since it's output is exposed to alternator current.

Schumacher was very specific on the latter statement and said all warranties would be voided if one was returned with this kind of damage. They also said other brands of chargers used similar technology as theirs and would probably also burn out.

My solution to having an automatic float function is to have the charger AC input run through a 12v relay controlled by the ignition switch. This way, any time the engine is started the charger will not function and will be protected from the alternator output.

This will ensure that the generator can be run while traveling and for manual float operation, will protect you from forgetting to pull the plug on the charger before engine start up.

The charger should be at least 2 amps output for start batteries. Water protection will be necessary for the charger (if in engine compartment)as the largest waterproof one I can find is only rated at 0.8 amp output and is a Deltron product, Some other of our members may know of a waterproof 2 amp unit- your post would be appreciated.

I built most of the relay setup tonight placing it in a 4x4x2 waterproof PVC electrical box. I used a small 12v Potter and Brumfield relay with contacts rated at 5 amps - more than enough for the charger. I will post pictures when completed.

continuous 2A will cook the water out of your batteries

Most 2-5 amp battery tenders, those chargers designed for tending the battery,do not put out 2-5 amp continuously. They cycle on and off or the better ones reduce the output current to a safe level once the charge voltage reaches a preset level.

I have one (2 amp) on my Chassis batteries and it works fine. I do not disconnect the tender at all from the batteries. The starter or alternator should not hurt it.

I will get the name tomorrow.

About leaving the charger connected and starting the engine, I often put the charger on the start battery for a while if the Euro has been sitting for a while, then start the engine, go outside and disconnect the charger. Am I risking harm to the charger in those seconds that the engine is running and the charger is still hooked up?
I'm talking about a cheap 2-10-50 charger.

My charger is a "programmed" type and only puts out the 2 amps when the batteries require it. Otherwise it only provides the current to maintain and will not boil the batteries dry. The 2 amps is to get the charge back up in a reasonable length of time.

Ed- I would like to know what charger you have- if its a Schumacher product, this would fly in the face of what their tech staff told me. In any case, I am still going to install the relay to be sure. If I don't, with my luck, the SP3 will burn out.

Clint

Just as an afterthought to my last post,it might be that the alternator might not hurt the charger under normal circumstances however if the voltage regulator on the alternator should fault and create a transient spike or high voltage condition, then the charger could malfunction. Schumacher would want to cover themselves on this situation, hence their policy statement.

Comments?

Clint

quote:

Originally posted by Clint Webb:
Schumacher would want to cover themselves on this situation, hence their policy statement.
Clint

I think you nailed it. Chances of a spike that high are slim but they want a way out I guess.

Alternators generally do not produce any spiking. They may fail an the voltage may rise much higher than normal, even so that voltage will be limited by the battery to some extent. If the batteries are old and weak, their internal resistance will be higher and will allow some higher voltage to exist.

The started is the main source of spiking on the +12VDC system. Again the spiking will be much worse if the batteries are weak or old for the same reason as above.

Most battery chargers have some sort of over voltage protection built in. This is not to say they can not be damaged but it is very unlikely.

Yes, most statements are made by the manufacturers to CTA!

My Xantrex inverter charger is on the house batteries all the time including the times when they are paralleled with the chassis batteries and the alternator is charging. If I were to use the "aux" battery switch, then they would also be paralleled during starting. No issues in over 125K miles and LOTS of battery usage!

Ed:

Sorry took so long to reply. Thanks for your comments and detailed analysis.

I am aware that our RV converters are designed to be connected to the batteries during periods that the engine is running. I am sure that their design is arranged to accommodate that. In theory, the smaller chargers should work the same way. I made my inquiries to be sure before placing my particular brand of unit online. The replies on this thread indicate that, based on experience and regardless of what the manufacturer states, it is relatively safe to treat the small units like the larger ones which generally means our members need not be concerned with their small chargers(programmed smart units only).

Relatively safe doesn't cover "odd" experiences(which I seem to frequently run into) and for that reason, I will still install my little bypass relay. It boils down to a personal preference- and also I have the thing built.

Clint

Another option is to use a DC-DC charger like the LSL Products TLS-OEM Ultra Trik L Start 5 Amp Starting Battery Charger Maintainer With Out Battery Clips. I believe it may be used on Winabago's. I got mine from RVUpgrades.com for under $40. This unit charges the chassis battery from the house battery, seems to well built and is quite easily installed.