Another battery question ....

I was wondering this myself, i have a second battery in the tub of my rodeo, running a 40l fridge. Specs of the fridge say input 10.8 to 14.2v. I thought thats pretty standard as alternator fluctuates a bit with revs etc. My concern is that my 3 stage battery charger can put out nearly 16.5v during absorbtion mode. I didnt want to risk jamming too many volts into the fridge incase it blew. I wonder just how sensitive the electrics of the fridge are??? i doubt alternators would ever spike that high. i could be wrong tho.
it would make it so much easier if i could just plug my charger in without pulling the fridge off and connecting 240v to it to keep it cold. I also wonder how effective the charge would be, ie. if the fridge is taking too much current away from the battery and not allowing it to charge properly but i guess as you said no difference to the alternator charging it, and also for the 3 stage automatic chargers, would the load of the fridge confuse the charger and send it to absorbtion stage pre-maturely or hold it in bulk mode for too long. and if so, would it even matter.
I dont have the greatest understanding of all of these principles, im sure some of you do tho. Would be great to get some feedback.

Thanks Fugwurgin

Hi Bomber,

yes it'll work .... but:

Because your solar input is not controllable unless there was a solar charge controller/regulator in between the panels and the battery....your battery will either get over or under charged.
Good solar charge controllers measure the currents going in and out of the battery by a shunt (chunky resistor with small resistivity in line with the battery wire).
They also take care of the maximum charge voltages, some of them offer temperature compensation.
You'll almost certainly get a longer life out of your battery by using a solar charge controller.

@ Fugwurgin,

you probably are aware of this - 16.5V absorption, I hope you don't have a VRLA AGM or Gel type battery because they don't want to see this high absorption voltage ever. It's something for flooded lead acid types.

As to the fridge input voltage exceeding the specs by a bit over a Volt - can't see a problem with that really as the electronics inside will have its own mini power supply which takes care of any spikes etc.
Speaking of spikes, alternators can generate some noise on the output due to the rectifier/regulator diodes not acting fast enough.
But this is usually absorbed by the battery which acts like a big fat voltage smoothing capacitor.
Nothing which would upset the fridge as it's been designed for this kind of environment I would imagine.

Your second concern of the multi stage charger holding the battery in absorption mode for too long is definitely valid.
But only if the fridge runs continuously, and/or there are other loads connected which would push the current up to higher than 10% of your specced charger current.
If no additional loads, then the moment the fridge cycles off while the battery charging current has tapered off sufficiently, the charger will switch over to float don't you reckon too?

Best regards, batterymeister

Hi batterymeister
i have a Supercharge Allrounder, the guy from battery world sold me the (what he told me) was the appropriate charger. It has the selectable battery types, he told me to make sure it was switched to calcium as these flooded batteries contain small amounts of calcium and need a high voltage to charge it up and said an alternator would never fully charge it. I hope he is correct, but i have to say im not happy with the Allrounder battery, taking it back tomorrow to have a chat with him (but thats another story).
And what you say about the charger floating with a load makes a lot of sense.
Thank you for your reply.

no worries fugwurgin, glad to be of help.

Before seeing the battery dealer, arm yourself with a little battery knowledge:

A great majority of small and medium size lead acid batteries sold (flooded, AGM and GEL), have their grids made from lead calcium alloy.

Flooded ones are more prone to stratification, self discharge, sulfation and shedding of active material compared to AGM.

These affect every cell inside the battery to a varying degree, leading to cell mismatch in the series string.
If this mismatch is left unchecked, it'll lead to even more mismatch.
It's a bit like wiring small and large capacity batteries in series and trying to charge all of them in one go.
The easiest way to counter this problem is by heavy overcharging the battery, which ensures every single cell will actually receive a full charge.
This high voltage charge is called equalisation charge, which de-stratifies the electrolyte and fully charges every single cell, but on the other hand this leads to more water break up into its gasseous building blocks.
Because most flooded batteries offer service caps, you can top them off with distilled water occasionally to make up for the loss.

For this reason, NEVER buy a 'maintenance' free flooded battery which can't be topped off because it's been made so cheaply that even the six threaded caps would have added too much cost to the manufacturing process.

Enter the world of VRLA AGM batteries, and never worry about having to equalise and refill them because they don't stratify and usually aren't very susceptible to cell mismatch. In fact, the boost charge voltage of around 14.7V is already high enough to do a mini equalisation on all cells. Their little gas generation is taken care of by a process called recombination under slightly elevated internal pressure.

AGM batteries offer superior deep cycle capabilities, less self discharge/sulfation, and can accept more charge in a shorter time. They usually also last two to three times longer than a flooded design.
Their only drawback is that they are a bit more picky about being recharged properly.

Thus, get a charger which matches their charging requirements as closely as possible to get the best out of them.
Remember your (VRLA AGM and GEL) battery is only as good as your charger.

Use your alternator to float charge them at 13.8V while on trips, and give them the occasional boost charge with the right mains charger.
Keep the battery as close as possible to 25 degrees when boost charging, even if the charger offers temp compensation.
Disconnect them when back home and store them under the house or somewhere else cool.
Permanently float charging them @ 13.8V will cause dry out in summer when the temperature corrected float charging voltage could be as low as 13.3V.

If you'd like to find out more about this perfect combination, stay tuned.

Best regards, batterymeister

Hi batterymeister
you certainly seem to know your stuff. Thank you
Are you familiar with the Allrounder battery? it is maintenance free, no way to top up the cells, it has a coloured "eye" to check the condition. it can be unscrewed to check fluid with a hygrometer??? but dont think you can use it to replace fluid with water.
I was told the Allrounder was a good comprimise between a deep cycle and a cranker.
I was using an old n70z supercharge cranking battery that would no longer start my GU patrol so i chucked it in as the second battery and was just waiting for it to die, it worked a treat for 18 months for the fridge 24\7. I was going to just chuck it out but glad i didnt. But when it did finally die this is when i bought the allrounder. about 3 months ago.
I am thinking now without forking out the money to buy an AGM would be to just buy another N70z cranker for $140 and not worry too much about it. Perhaps thats just creating a false economy as id probably have to replace it often. anyhow, im just a little frustrated with the allrounder.
i put it on charge the other day, was down to 11.97V left it on charge overnight, floated the next morning @ 13.8 (like it should have) left it there disconnected from charger for a day, went to hook it back up in the car and out of curiosity put the DMM over it and it read 12.34V, thats a big discharge in just one day without it running anything. Coloured eye is still green?????

These AGM batteries you talk about. how low can you "accidentally" run them down to? my fridge has a cut of of 10.8V which is too low to be of any real help to the battery. Are the AGM's more forgiving to lower discharges.
Im sorry if ive hijacked the thread, but i found ur last couple of replys very helpful.
Cheers.

Ahhrrg, it's one of those non-servicable flooded ones, 'maintenance free' is a total misnomer really....
I think the coloured eye is an integrated hydrometer. In your case this might be used for topping off just this one cell, while the other ones have to be left to dry out over time - not good.

Your supercharge still being able to run the fridge for a while is a good example of how to squeeze the last drops of usefullness out of any battery. Generally, batteries are considered to be at the end of the road, when their capacity has dropped to 60%.

I agree with you on the false economy of buying a cranker to power your fridge with. The cranker will have a hard life if you regularly discharge it below 20% or so - it could start to shed active material early on in its life and loose capacity quicker than in a cranking application. For another 100 bucks or so you'd be able to find a good quality deep cycle 100Ah battery which will give you around 1500 cycles @ 20% DOD, or around 300 cycles @ 80% DOD.

12.34V OCV is about 40 to 50% discharged. I recommend to put it on the charger for another 24 hours and check the voltage 24 hours after it's stopped charging. You want to see 12.6V OCV at least.
If it doesn't come up to this voltage the acid concentration could be low - but unfortunately you can't do much about this...

As to the fridge cut out voltage being 10.8V, this is not 100% DOD, but very close. AGM batteries allow for a 10.5V cut off, and if its a good deep cycle, you can do this a couple of hundred times.
But you better make sure the battery will receive a 100% recharge between these rock bottom discharges.


I've heard of fridges with adjustable cut out voltages - have you checked yours? If not adjustable, read on:
One trick out of the electronics tool box is to wire a bridge rectifier in series with the fridge.
This will effectively raise the cut off voltage to a more acceptable 11.6V.
The down side of doing this is, you waste about 8% of battery capacity in the form of heat in the rectifier, resulting in less run time for the fridge. But this could be cheap insurance as you will gain maybe another 100 cycles before it reaches its end of life.

FYI, you could buy a 35A bridge rectifier (less than 10 bucks) at Jaycar, mount it on a 100x100mm aluminium heat sink, connect the two ~AC terminals together which form the anode, and the + terminal forms the cathode.
So the circuit goes like battery plus, fuse in/out, anode, cathode, fridge plus, fridge minus, battery minus.

Best regards, batteryvalue

The Danfoss BD35 and the BD50 were originally designed for 24v systems , low voltage ,under 12.4 v, is the major source of percieved failure of waeco ect that use the Danfoss compressors.

The fridge will be running on a duty cycle of probably 50% (max) so half the time it's actually not even running and the battery will be getting the full charge anyway.

The other thing to consider is the battery is acting like a huge voltage sink, the power from the charger will be depleted to some degree by the battery anyway and will only be going in full to the fridge when the battery is 100% charged and not accepting any charge at all, which never happens anyway.

Maîneÿ . . .