sorry fellas.....battery question

I’ve got 2 Century Marine Pro 730's under the bonnet and a Lifeline AGM in the caravan.
I wouldn’t put an expensive AGM under my bonnet as the heat would shorten its life and I don’t believe I would get value for my dollar.

I never had any intention of putting an AGM under the bonnet but really wanted to know if an AGM or Gel would be best in the van or whether to go with a Marine Pro, bearing in mind that I'll be putting two of those in the car eventually. ???

Steve,

What sort of voltage are you getting back at the van battery with everything plugged in and the engine running (fast idle)? Do this measurement with a half discharged van battery, resting voltage about 12.0V. Compare it with the voltage up the front.

Without a dc-dc charger, the success of your system depends on good cabling and connections.

How long is the cable run from your engine compartment to the van battery? Whatever it is, it will be long. If the cable sizing is inadequate for the length of the run then you will get excessive voltage drop, even given the higher starting point from your diode booster, your charging voltage AT THE VAN may be insufficient and your van battery may suffer in the long term as a result. (Could this be why the present battery is dying/dead?) If that is the case you could re-cable - I would suggest minimum 6 gauge including negative runs to chassis earth points. Or it might just be simpler to go the DC-DC charger route.

Another thing to consider is that, assuming you're going to put a deep cycle battery in the van, many (but not all) deep cycle batteries don't like the high charge currents that an alternator can deliver. Typically they like a max charging current of 10 to 15% of their C20 amp-hour capacity. ie, max current for such a 100Ah battery would be 10 -15 amps. So if you have the voltage at the van, choose a battery that will take the alternator output, and if you go the dc-dc charger route, match the charger to your battery.

Cheers

This is not questioning anyone's methodology, just a question.

It sounds like Steve has a similar setup to what I run. A cranking battery being the first in line for charging from the vehicle alternator. Then an AGM connected through VSR. Finally an Anderson plug with appropriate size (can't remember exactly) wiring to feed 2 AGM 105 AH batteries in the van. To me the vehicle alternator should sense the batteries being charged up and as the battery internal resistance (state of charge) rises the charging voltage increases and the amps taper off.
Not sure this is the ultimate way to go but seems to be working so far. All the AGM batteries are about 4 years old and seem to do their jobs OK. I replaced the cranking batteries about a year ago even though the old ones worked ok, just to be on the safe side.
When a 240v source is available, CP or similar I leave the van and vehicle connected so that both the van's and the vehicle batteries are charged by the van's Smartcharger.
With the vehicle running I get 14.4v at the vehicle batteries and about 13.8v at the van's batteries, so a volt drop of around .6v through the wiring. This after a good run of say 4 hours or more.
The vehicle is a pretty old low tech no smart alternator or computr involved jobby.
Cheers
Pop

Thanks Frank. I think the cabling is 6 gauge but need to check this as it was installed by previous owner. I thought it was working ok beforehand but if the cabling is lacking, I thought it would be an easy and relatively inexpensive job to run a thicker cable through and could only improve it? I was quoted $1000 locally to install a DC/DC so thought the current setup would be ok and certainly cost less. Would I still need to improve cabling with if I install a DC/DC if it is less than 6 gauge?

Arrived back at 1 pm today after a 3 hour drive and I have just (at 5.30) taken a reading with the multimeter @ 12.50. I then put a light on for a minute or so and it is still reading 12.50 so I'll check again tomorrow. Then probably give it a proper charge. Might be life in it yet? Maybe just left something on and ran it down?

The first question I ask is.

Where is the battery in the van located...and how well is it ventilated.

If it is in a poorly ventilated location I believe there is a case for an AGM or other high status battery.

But if the ventilation is good and the battery box is properly designed to contain and resist acid spillage or fumes.....I do not believe there is any advantage at all in AGM....appart from status and draining your wallet..

If you are not averse to activly maintaining a battery Trojan, wet cell, deep cycle, screw top, batteries are worth a look...they are a "propper" deep cycle battery....but ya ventilation better be good.....and they will need regular topping up.

a half way option is some sort of sealed, wet cell battery......in the top of the market they have all the advantages of AGM and pretty well non of the drawbacks and they will be cheaper.

A you may know....I and a few others are fond of the modern sealed marine batteries.......when a good marine battery that does have better deep cycle performance than standard batteries and at half the price of AGM.....they realy are worth considering.




as for the dc to dc charger........it may or may not provide an advantage.

If you go for an AGM with a low maximum initial charge rate you need a dc to dc charger purely on the bassis of current limiting.

as for a $groilla to install a dc to dc charger into a perfectly servicable existing system...HELL.

You can buy one of the top brands DC to DC chargers for between $300 and $500, depending on the model and how well you shop ( lesser brands can be had for as little as $200)........so what takes 5 hours...( one assumes $100 an hour).

Seriouly it aint rovcket science......mount the dc to dc charger unit....reterminate the incommong battery cables and fit to the dc to dc unit and make up a pair of cables between the dc to dc unit and the battery.......maybe insist on checking or reterminating the other connections... 2 or 3 hours should pull it up tops.


cheers