Charging AGM Batteries

I use an Arrid Twin Charge for my charging needs when running the vehicle. This uses a reasonably light 12 volt cable (no need for a Anderson Plug) and has a 20 amp output. It charges my 120a/h Fullriver AGM very effectively.

I am about to install a 2nd unit in the camper trailer for the 2 Supercharge Gold Series batteries in there; at the moment I am just relying on the trickle-down effect from the AGM, which seems to work okay too. So, effectively, the Arrid unit is presently charging 3 batteries when I am towing my camper.

I have a second alternator ( about 65 Amp) on my Range Rover connected to my 95AH AGM.
I find that it takes about 2 hours to get to 80% and another 2-4 to get to 100% from effectively flat (say 11.8volts)

I can tell this from my digital meter which reads about 13.7 Volts for 2 hours then slowly rises to 14.2 over the next 2-3 hours.

So I think it will take longer than that for a second battery on the one alternator, although if you have a late model with a 125amp alternator, it may be just as quick..
I guess it depends on the initial available/surplus power over and above your ignition,aircon,lights etc etc in the high charge phase,as I would think the taper phase would be similar.
Regard sPhilip A

Thanks very much for the input guys - appreciate you taking the time to reply. Have a good one.

we run 2 X 100 amp hour AGM's in the camper trailer fed from the vehicles alternator via an Andersen plug. However this power is fed through a shunt monitored by a Plasmatronics PL20.

The PL 20 records amps in and out over a 24 hour period...By using the shunt the PL20 then can allow for amps in so as when we make camp and feed the PL20 with solar panels it can take into its calculations the amps already topped up in to the batteries it is managing. Cable size from the alternator is critical....unless you calculate that correctly your alternator will still only deliver a trickle of amps at the other end.

Unless you have a battery charging system device that isolates your batteries and charges them as an independent system then once the start battery is recharged then all you are doing is connecting all your batteries in parallel.....you will never get a true state of all batteries at 100%

I have been told that as a rule of thumb each battery connected down the line will be around 10% lower than the one up the charging line.... how true that is i dont know

All I can say is that the more people you ask the more differing opinions you will get. Our system works for us.

Try this site and it's links www.fridge-and-solar.net/agm.htm

No question: you should buy the most expensive three (or 68) stage charger available – if it costs less than $600 or $700 it’s rubbish.

Next step: when you want to recharge in the field you _must_ fit a second alternator to your vehicle (with, at least 160A capability) and ensure it has krypton brushes – anything else is useless. It probably needs 14 safety certifications too!

Mike Harding

PS. I’m currently being paid good money to test various lead acid batteries to ensure they comply to Australian Standard 4029 (iirc) but I recharge my 88Ah AGM from a $98 Bunnings generator - however I’m sure many on this forum will soon post to tell me I’m doing it all wrong.

PPS. This is a satirical post.

G'day mate,

I have a Fullriver 120a/h in the camper as well as a Xantrex Battery monitor that monitors all amps going in and out of the battery.

After driving the max charge I have ever got from the car's alternator (via anderson plug) is 99.7% so I think you can safely assume that you will get relatively close to full charge from your cars alternator.

Hope this helps.
Cheers,
Paul.

I am all for a spare cranking battery to run the fridge. They can charge up quick and the amount of current draw overnight is not all that much as to cause the battery to have so much loss of overall life. In the event of one dying as they are bound to do sooner or later, I can use the other to run the car. Just got to see that it gets charged up every day so the percentage of charge dont drop dramatically. Charging from the car motor would take a lot less time than an agm or deep cycle = less idling the motor on site.

Of course if you are running a lot of other current hungry devices and not running the motor every day then deep cycle with external means of charging I would agree with. I am not on the road for long periods out of each year or running a live aboard yacht so I look to the comparative cost/efficiency chart and conclude I would be better not spending on expensive 240 volt chargers and batterys.

Horses for courses.

Owen

I found this webpage a while ago. It answers just about every question about batteries and charging that you could think of. I have spent hours researching the subject and this site appears to have summarised everything with the best accuracy. Have a read and see what you think.

Site Link

Said very simplistically:
If you put more 'power' into a battery, than is presently in the battery
then it is in fact being charged.

It does not matter if the 'power' is coming from a $2,000 3x (or 27) stage battery charger, a solar system or the vehicle alternator/regulator.

Scenario:
If the battery is only holding 12.2v and your alternator/regulator is capable of putting out 14v (~every alternator/reg will) then it will charge the 12.2v battery to a higher level, it will in fact charge the battery fully - 12.66v.

However, if the battery is 'faulty' or is attached to an inadequate wiring system, then it may not hold its full charge, and then will appear to not be fully charged.

Mainey...

We have a 260AHr Fullriver in the back of the Patrol with current shunt and amp meter to monitor charge progress (only way to check really IMHO).

On a recent run from Rollestone (North of Townsville) to Mackay (approx 6 hours @95km/hr) I noted the following (battery was at 12.25V which equates to approximately 65% capacity):

@ idle - 750rpm, there was 26 Amps going into the battery.
@ 1800rpm there was 55 Amps going into the battery (max it could draw from the alternator either because of battery design or alternator capacity)
1. From time 0hr to 1hr, it went from 55 Amps to 42 Amps (draw)
2. From time 1hr to 2hr, it went from 42 Amps to 31 Amps (draw)
3. From time 3hr to 4hr, it went from 31 Amps to 23 Amps (draw)
4. From time 4hr to 5hr, it went from 23 Amps to 15 Amps (draw)
5. From time 5hr to 6hr, it went from 15 Amps to 9 Amps (draw)

When I worked out the average for each hour (start draw + finish draw divide by 2), I came up with 133 Amps of charge going back into the battery over the trip. Considering there are losses due to heat etc when charging, I considered the above to be reasonably accurate.

So to fully charge the battery I would probably have to drive another 1-2 hours. A battery will only draw as much current as it is designed ie. having a 200 Amp alternator is no better than a 100 Amp one if the battery will only draw a max of 55 Amps.

If anyone wants to know what their battery is doing, buy a current shunt and digital amp meter (cost me $20 all up) and see what current the battery is drawing. it will get less and less as the voltage rises and its capacity increases.

Regards
David