parallel car battery

jamie: sure why not i would put in a 40 amp diode from pos of large battery to the pos of the small battery, auto mech

As long as both batteries are connected together, they will charge together, and drain together. Use battery cables to join them that are sufficient to carry at least the maximum current drain, or charge rate. Do not deliberately drain the combined batteries too low, because the charging alternator will have to work at a high charging rate for a longer period of time, and may overheat.

What is the purpose of the second battery? Is it to start another motor? For all other use use some form of circuit protection.

If it is to start another engine, or to increase the starting power of your existing engine you will need to use heavy starter grade cables and be very careful with routing to avoid chaffing the cables.

Better questions will yield better answers. If you are more specific we will be able to be more helpful. Good luck.

I'm not so sure that that is a good idea. The two batteries will charge and discharge at different rates and will produce slightly different terminal voltages as a result of their different capacities. Without a charge control that drops each battery out when it gets to full charge, you will get either one battery (the 100Ah) only part-charged, or more probably the other will get overcharged. Overcharging will kill off the battery in short order.

Check with your local boating supply company. Most boats have two batteries; one serves as a main battery, the other a back-up in case your battery dies while on the water. They sell switching mechanisms that allow you to run off one, the other, or both batteries. I believe they incorporate some sort of diode mechanism that will allow the off-line battery to charge even though its not actually being used. Its been years, so I don't recall, but its as good of a place to start as any.

There are issues with both of your suggestions. The switching mechanisms present an obstruction to the current flow in the starting system.

The diodes that allow the batteries to be isolated from each other, drop the voltage between the alternator and battery by 7/10Th's.

A better question would have helped, but.............

Automotive supply stores also sell Isolaters that are designed for exactly this purpose. $30-$70 US.

Battery isolators are workable devices for low current applications that do not have voltage sensitive equipment. Isolators for alternators over 100 amps get more pricey. They will also send out 7/10 volt lower voltage to the batteries. If radios are sensitive, they can cause problems when first starting the vehicle at night with lights on. This is may not be a problem, but we may never know. The OP has gone silent.