Re: Battery Isolator...Should I use one in my two battery set up???
I'm sorry to hear about your grandfather.<br /><br />Isolation is always safe.<br /><br />I'll go into a little more detail, and you can make up your own mind.<br /><br />As I posted above, if you hook to batteries together in parallel without isolation, you force the batteries to drain exactly the same. Well, starting batteries and deep-cycle batteries are not designed to drain the same.<br /><br />Deep cycle batteries have more amp-hours (which means they actually have more energy storage), but this design also gives them higher internal resistance than starting batteries. This is why starting batteries have higher CCA, because of the lower internal resistance.<br /><br />So, the lower internal resistance of the starting battery will actually make it supply slightly MORE energy when the two batteries are connected in parallel. It has less energy storage, and is actually supplying slightly more energy. As a result, it will drop voltage faster than the deep-cycle battery.<br /><br />So the question becomes, what happens when one battery drops voltage? Lets pretend it's a fairly large difference, like 4 volts. The internal resistance of the batteries is very low, just a few milli-ohms. Let's say it's 10 milli-ohms, even though I think that number is probably high. By ohms law, current equals voltage divided by resistance. When you have the two batteries connected in parallel, you have a closed loop, a complete circuit. 4 volts divided by .010 ohms equals 400 AMPS! By hooking two batteries of different voltages together in parallel, you can develop HUGE loop currents.<br /><br />With a difference of 4 volts, the deep cycle battery is pumping 400 amps into the starting battery. One, this could cause damage to both batteries, and two, this severely affects the efficiency of the deep-cycle battery. Now, granted you are not likely to develop 4 volts of difference if the batteries are always connected. But then, look at what is happening. The deep-cycle battery is charging the starting battery. If half of your energy from your deep-cycle battery is used to charge your starting battery, then your deep-cycle isn't going to last very long.<br /><br />As well, there are other dangerous "times" with this scenario, even when both batteries are fully charged. Starting your motor usually requires large currents, and current flowing through a resistance creates a voltage drop. Because of the difference in the internal resistance of the two different batteries, they develop a voltage difference during large power drains (such as starting). This again can create large loop currents.<br /><br />Now, the starting and deep-cycle battery is the extreme case because they are radically different. But think about it, each and every battery is slightly different. They have different capacities and internal resistances. Even between two batteries of the same size and type from the same manufacturer, but one battery is older than the other, because internal resistance increases with age. If both batteries were exactly (and I mean exactly) the same, then there would be no danger at all. The risk comes from the difference between the batteries. The greater the difference, the greater the risk.<br /><br />With an isolator, yes you lose some efficiency across the isolator, but you have eliminated all risk.
