Mains battery charger to keep a battery that's in charged?

A Smart Charger designed for caravan / motorhome leisure batteries should do the job.

In essence, many motorhomers in particular ( including myself) do just this when they keep their motorhomes on ‘hook up’ ( plugged into the mains) while on their drive. The modern smart chargers are designed to follow the correct charging profile, including a ‘float’ stage. They can also work with a solar charger.

They are also used while ‘onsite’ and the Motorhomes or caravans are on hook up. In modern leisure vehicles, the lights, even the TVs, tend to run of 12v. In ours, only the microwave uses 230VAC ( the heating is dual gas or 240, ditto the water heater. The fridge is a 12V compressor one. ) As a result, vehicles have leisure batteries which are ‘float charged’ if mains is available.

The down side is, motorhome kit carries a price premium. You could try looking for a second hand one.

A lot of people are changing to Lithium batteries and changing their chargers. You may be able to pick up a perfectly good charger which lacks the Li profile.

Yes, but how do I find one? I.e. what keywords in my search will produce hits for just this sort of charger?

Leisure battery smart charger lead acid

Those should help.

Popular makes are CBE and Sargent.

A Sargent PX300 was very popular on a number of UK built motorhomes and, while it will cope with Lithium, many people upgrade. You may find a second hand one.

Likewise, the some older CBE ones, while they will charge a Lithium, people often upgrade to one specified for it.

Victron is a very good make but pricey.

On eBay that brought up one result that was specifically **not** for lead-acid batteries! :-) I've been trying searches with similar words for quite a while with not much success.

Quite! That's why I'm trying to find alternatives.

I'm not sure that one will do what the OP's after. At least, in the case of the later PX310, the manual states:

I suspect you aren’t interpreting the results.

Many Leisure batteries are Lead Acid, just deep cycle ones, etc. Until March, we always had Lead Acid leisure batteries.

True, many people are converting to Lithium and newer motorhomes etc tend to come with Lithium leisure batteries but not all.

He seems to want exactly what happens when a motorhome etc is used on ‘hook up’ ( parked and connected to the mains).

Chargers on motorhomes etc are, essentially, fit and forget- or designed to be. True they have displays etc on the control panels but most owners have little idea what is going on.

Yes, (OP here) I do. It's actually for a 'UPS' at home to keep the internet ONT and router plus the VOIP phones working if we have a power failure. I have a (retired from boat) 12v 100Ah leisure battery providing power to the router, ONT, etc. I thus need a charger which will keep the battery charged while it's powering the various devices.

The total drain is probably only about 3 amps so I don't need anything as powerful as most boat/motorhome installations would expect.

As on my boat, solar panels feeding an excellent Votronic MPPT charger that keeps the batteries charged when we're not there and, in the summer (when we are) it provides much more power, as required.

Votronic do make motorhome mains chargers but they're over £300 for the smallest one (15 amps which is much more than I need anyway).

I just need a fairly low power charger (say 5 amps) that will watch the battery voltage when it gets to maintenance/float mode and keep the voltage a a suitable float level.

And that's the issue: you need a device that will charge the battery

A battery charger is normally a current-driven device now, and it monitors the battery voltage. If an unspecified load is also drawing current from it, it cannot know how much is going into the battery, and therefore cannot monitor charge state and battery condition. Or at least, it will, and will come to incorrect conclusions.

So a two-output device is necessary, with automatic switching of battery to load when the mains goes away. I did this on a tiny scale for a handheld remote controller, with a battery and also external power input. At this kind of power level, it was trivial, with just a couple of diodes to switch the battery to the load. But with a load in amps, probably proper MOSFET switching will be necessary. The market for such a device will be much smaller than for a straight charger, hence the exorbitant price.

I've used these dumb auto-switchover PSUs for a slightly similar application:

Two outputs aren't necessary (although that's one way to do it). There are lots of (mostly too expensive for me) chargers out there that cope perfectly well keeping a bettery charged when there's a load drawing current at the same time.

I have one myself, the solar panel powered charger on my boat keeps the batteries there well maintained whether or not we are on board using 12v current or not.

It's not **that** difficult, all the charger has to do is monitor the battery voltage and keep it at the 'ideal' 13.5 volts or so by varying the current it outputs (the charger that is).

Ah, thank you, I did look at something like that when I was thinking about this earlier. I'll take another look and think some more! :-)

An important detail, which you have missed, is the usage current. What are you powering from the battery?

I have multiple chargers, some just output a constant voltage, some are smart. The smarts, need a button to be pressed, to start the charge process, and will need the button pressed, after it has tripped due to charging the battery fully.

Of the dumb chargers, one is able to output up to 5amps, at a constant

The first one, is permanently connected to my cars battery, and is auto-turned on, for 20 minutes per day, to keep that charged.

A surplus desktop PSU, can make a good charger/power supply, if you are up for modifying the 12v fixed voltage output. Often a matter of changing a resistor, on the potential divider, of the output sensing circuit. I modified an ex-BT 50v 50amp PSU, to output 13.8v, by that method.

Go to a good car accessory dealer, and see what they have. You are looking for a simple, dumb charger, with a fixed voltage output, and no buttons to start the charge. For reliability, you should aim for one rated to output a continuous 6amps.

Likely, the fixed maximum output voltage might be to high. A simple workaround for that, would be to insert diodes in series, with one of the charge leads, to reduce the voltage. I would suggest dropping it, to around 13.5v to 13.7v. Each diode, will drop the voltage by 0.6v, but the diodes need to be rated for 6amp.

All you need, is a very basic charger, with a fixed voltage output, and a more than adequate current rating, to cope with the full, constant load, whilst on charge - nothing really complicated about that.

Now you've told us more about the likely load etc, I'd suggest something cheaper and simpler- if you have access to a voltmeter.

Buy an adjustable PSU off Ebay - look for one rated at around 15V and

Put the diode in series with the positive output of the PSU with the 'arrow' symbol 'pointing' towards the battery +ve.

Before turning on the Power, turn the output pot on the PSU down.

With the voltmeter across the battery (NOT the PSU) and set to a range suitable to measure around 14V, turn on the power.

Now adjust the output pot until the meter reads 13.5V.

Check the voltage from time to time, over the first few days. If the needed adjust once the battery charges (if needed).

The diode is to prevent damage to the PSU in the event of mains failure- some PSUs don't like an external voltage being applied. It may not be required, it is a precaution.

I've used the above approach in the past (although in higher current situations) without problems.

• Stay in the 'float range' and the battery shouldn't gas etc. Staying at the low end allows a margin of error.

Ideally though the charger should, it intervals (like weekly) fully charge the battery up to 14.4 volts. A totally constant 13.8 volts isn't good permanently.

The Votronic chargers I referred to do a weekly (or every 5 days or something) re-charge to 14.4 volts.

Running at that 13.5/13.8 volt level **permananently** isn't good for the battery, one needs to do a refresh to 14.4 volts occasionally to keep the battery in good shape.

What I have always understood, is that charging to 14.4v, squeezes the last bit of charge into the battery, at the expense of additional battery wear. Fine, if you plan to need to use that extra immanently, but not necessary to simply maintain a battery.