problem maintaining leisure battery charge

[veedub565]

I might be being a bit thick here, but perhaps someone here knows what's happening.

I have two 12v leisure batteries connected in series to make 24v. I use them partly as a battery backup power source for a HF radio, and partly to power the radio when operating mobile. To keep the charge topped up on them I bought a CTEK MXT 4.0 smart charger. My understanding is that the CTEK should keep the batteries topped off and prevent them discharging while not in use.

I was rather surprised then to find that after only a month or so one battery was indicating it required charge. And the other, although still showing green on the indicator, did still take several hours on my MXS 5.0 charger. So, one battery discharged and the other not far behind.

Have I misunderstood what the CTEK maintenance charger is meant to do?
I want to maintain the batteries at a high state of charge so they are ready to go if the need arises. is this a bad way to maintain leisure batteries ?

The HF radio is always connected to the batteries, it operates from mains power normally, and I don't think it draws any power from them unless switched to operate from battery power source.

[floobydust]

I would look at two things.
First, (in general) these smart chargers can have dumb software and screw up with auxiliary loads, which confuses them.
For caravan/RV use some brands seem to get stuck in a float or pulse maintenance mode and fail to restart charging as required in the presence of aux loads. So they chronically undercharge.
There are some brands of "smart" chargers that RV users are finding the batteries have a short (1-2 year) life as they are left running in a (float) maintenance mode.
The CTEK chart seems to say it will restart charging though "if voltage drops" for the 12V models.
It doesn't know your battery size, so a few amp (aux) drain if it's waiting for the battery voltage to come up, so that it can exit that charging phase and move on - it never completes. The F/W does have a timeout which overrules and it will either abort or move on to the next phase.
I'm not 100% familiar the CTEK "the smartest charger in the world" but would get more info on yours.

Second, these chargers have not so great chinese electrolytic capacitors that can't last. The sealed box keeps the heat in.
So I would take it apart and inspect the caps. It's easy to put on 10,000hrs on these chargers. It might have a hardware problem.

[Geoff-AU]

so you've got series batteries for running your leisure craft (boat or whatever) and only hooking up the radio to one of them?

One battery being discharged more than the other means you need to charge them separately too.

and really for this kind of thing you're probably better off with 2 dumb float chargers and keep each battery at float voltage. Make sure the dumb float chargers have 2-pin mains plugs and then they'll be isolated.