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Discussion Starter · #1 ·
I’m looking to run a 50Ltr fridge and I’m definitely no expert so all comments welcome…

Most Dual Battery Management Systems (DBMS) allow the auxiliary battery to charge only after the main battery is fully charged and isolate the auxiliary from the vehicles electric system when there is no charge. Items like fridges are meant to be wired to the auxiliary batteries.

Why can’t the auxiliary battery be wired in Parallel (PAR) to the main battery without a DBMS? The issues I see;
1. Charging – if vehicle is run for a long time – No difference
2. Charging – If the vehicle is run for a limited time – PAR both batteries are charged at the bulk charge stage before going into the (less productive) adsorption charge stage. DBMS the main battery goes through bulk and absorption stages before the auxiliary gets anything… which means less amp/hours would be put into the two batteries. PAR win.
3. Discharging – PAR the load over two batteries and gives each battery a slower discharge rate and allows more amp hours to be drawn from the batteries, i.e the slower a battery is discharged the more amp/hours can be drawn from it. DBMS the auxiliary battery takes the full load. PAR win.
4. Discharging – PAR spreading the load over two batteries will mean double the amp/hours available vs a single auxiliary battery. PAR win.
5. Discharging – PAR spreading the load over two batteries means the discharge from each battery is 50% vs one battery and less likely to reach a level of discharge that would damage the battery, I’m told that continuous discharging of lead acid batteries below 40% reduces battery life. PAR win.
6. Cost – PAR no need to wire a power socket directly to the auxiliary battery if your car like mine already has a socket in the cargo area. PAR win.
7. Cost – DBMS Redarc and other units are expensive. PAR win.
8. Flat batteries – You could end up with flat batteries with PAR or DBMS. A low voltage disconnect on the auxiliary could be used with PAR but im not sure if it could be used with DBMS. The one below disconnects at 12.1V (a car should start with 11.8V) and it has a manual connect option is you need to use the auxiliary to start the car. Cost is circa $100 plus postage. With this you should always be able to start the car.

http://www.ase-supply.com/Cole_Hersee_48510_Low_Voltage_Disconnect_p/ch-48510.htm

What am I missing?
 

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GQ Dual Cab. TD42Ti with fruit.
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Your starting battery is not designed for a deep discharge. Running it down will stuff it in a short period of time compared to a deep cycle battery which is designed for this kind of discharge. Also, it does not allow for the fact that you could flatten it, and not be able to start your vehicle. This is the main reason for it to be isolated.

I have used a solenoid to isolate my batteries for the last eleven years. Cost about $35 or something. It will always protect your main battery. It will not however do anything to control charging rates, as you have pointed out. I used to put my aux battery on the charger when I got home to allow for this. Never had any problems with main batteries, only gone through two in eleven years.

I think the proper isolators are the best option to be honest, as they will do a better job in the long run. If you are touring I reckon they would look after the aux better too.
 

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This means you would be using 2 cranking batteries. Dual battery you use 1 cranking and 1 deep cycle. Shortened life of batteries from discharge.
$100 plus postage for low voltage disconnect.
$155 plus post for Matson dbms
ALL PURPOSE BATTERIES PTY LTD
 

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Discussion Starter · #4 ·
GQshane and truckersgu thanks for your comments on crankers and deep cycle batteries.

True deep cycle batteries have solid lead plates and not lead/sponge plates that have more surface area and can give off the instant power that crankers need. I havent seen any sensiibly priced lead acid batteries in the auto market that are true solid lead plate / deep cycle batteries and specify the battery can withstand constant 80% discharges. I think most batteries in the auto market that are marketed as deep cycle are actually hybrids and will suffer if the are discharged too low.

GQshane, yep flat batteries are an issue, see my comments (no. 8) in the orginal post about the low voltage cutout.

truckersgu thanks for the link... those AGMs look good value and from memory AGMs can handle deep cycle and cranking.
 

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yeh i run the matson dbms and an ssb 110ah in my trol. Brought it from those guys. For the price of matson system i'd go for that. Comes with everything needed to hook it up.
 
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