Adding a Second Battery

i have a 2006 gmc sierra, 4.8 liter, just installed dual optima batterys, if your truck didnt come with plow prep. package you will need to buy an aux battery tray, battery hold down, bolts on the right side next to firewall which i can give you all the part numbers if you need. i went battery to battery, no isolator, the best wire to use is welding cable, i used #2 awg. if you have any questions or need pictures let me know and ill hook you up.salden77

I use an ordinary relay instead of complex equipment. The 150A relay cost AUD$15. It connects the aux battery whenever the vehicle is charging. I have a $7 50A circuit breakers at each end.    I also have a $25 under-voltage 80A latching cutout from the aux battery to its loads. I was using the same circuit (adjusted for 13.5V) for the charge-sense connection until I replaced it with the relay. Normally I'd just use a 30A setup, but a manual switch allows paralleling of my weaker front battery during extreme cranking (CB upgrade pending if required).

Hi! Simply use low cost schottky diode in line with the alternator, battery or line you want to isolate. Go look on eBay, digikey or other electronic store. They go from 400A to 0.0001A in a wild variety of voltage and price. You also can stalk them in parallel for more amp. The voltage drop is 0.3-1 volt but typically 0.7. If this lost is too much for you or you want a sightly higher voltage and a easily confuse with electronic. On GM, put normal diode parallel side up to side down on the voltage sense wire. and you have almost 1 volt more!!!-=-

A Schottky isn't required for its speed in an auto system - an ordinary diode will do. But if Schottkys have lower voltage drop and Germaniums aren't obtainable, the fine. But a diode will not prevent the original battery discharging into the second if the second fails. And the second is likely to fail if constantly subject to a lower charge voltage than the first (ie, via a diode). I recommend the relay option above. A cheap 15A or 30A relay should be fine for smaller batteries (I have used them for car batteries).    If the batteries are adjacent to each other, fuses/breakers are not required - except to protect the relay. It only connects the batteries whilst charging. The same relay else other battery interconnection can be used to parallel the batteries if required (ie, for cranking), Parallel batteries should be avoided. Keep in mind that a parallel battery halves the battery's reliability. PS - tweaking the "sense" wire works for most alternators except the D+ only types (AFAIK). Sense-wire boosters and other circuits are used by Rally drivers & audio buffs for maximum system voltage hence maximum headlamp brightness, audio power etc. (Once even maximum spark before CDIs!) Typically they would raise the system voltage to 14.4V at the battery where 14.4 was the common max battery charge voltage; higher where battery life was not as important.    It is the same reason many cook their batteries. If the sense wire is lower than the battery voltage due to intermediate loads (lights, wipers, brakes etc), the regulator will boot output an equivalent amount. A voltage drop of 1.5V can mean 15.9V at the battery - hence overcharging and possibly damaging other loads. (Hence why special care is needed with most two-wire systems - ie IGN & charge Lamp aka Ig or S & L etc.)

That's fine if you don't mind the independent and hence unequal discharge rates from the batteries and hence no load sharing. And any loads off the alternator handle the extra voltage (ie, as per Rally setups.) I was assuming a standard load/battery sharing arrangement whilst charging with equal system voltage. How does the dual diode cost compare to a relay (which I estimate to be $15 for 150A). And what about heatsinking requirements - are the diodes easy to isolate from the heatsink?

IÍ wanted to find a high amperage LATCHING RELAY.  Where did you find one

and what did it cost?  Wanted to keep my HID lighting ballasts off when I

start the engine then bring them on to avoid problems with them.

oldspark wrote: I use an ordinary relay instead of complex equipment. The 150A relay cost AUD$15. It connects the aux battery whenever the vehicle is charging. I have a $7 50A circuit breakers at each end.    I also have a $25 under-voltage 80A latching cutout from the aux battery to its loads. I was using the same circuit (adjusted for 13.5V) for the charge-sense connection until I replaced it with the relay. Normally I'd just use a 30A setup, but a manual switch allows paralleling of my weaker front battery during extreme cranking (CB upgrade pending if required).

"All paid jobs absorb and degrade the mind."

Aristotle

Latching relays do exist - the under/over voltage circuit I use has an 80A mechanical latching relay so that no power is consumed other than when changing state.    However I use standard relays with whatever circuit or connections are required. EG - I have a pump system that terns on at level-1 and off at level-2. It is 2 fluid level switches and 1 standard relay (SPST aka on-off), though I also use 2 diodes to ensure the switches never carry the load current. I'll have to re-read above to see what was is required in your case. And is it Odyssey that reckon no current limit applies and that you can exceed 14.4V (up to 15.6V from memory) PROVIDED some battery temperature is not exceeded? (Or was it Optima?) Normally AGM batteries are a constant voltage charge (say 14.4V) but with a current limit (20% of C10 or whatever....). I find it strange that a "lead acid" AGM would allow above 14.4V (at 25 degC which is normally considered the voltage for major gassing onset). Also strange that a battery would allow any charge current. Temperature increase is a indication of overcharging - it is not the preferred method of determining full or over charge etc - rather, it is an alarm situation (ie, stop charging!). How long does it take the internal heat to reach the outer casing? And if the casing is in a very cold environment, will this cool the casing hence masking high internal temperatures?    (For wet lead acids and semi-sealed - fine - you can insert a probe; but not AGMs.) Not that I care - I don't pay their warranty claims, and I don't have to prove to them that I never exceeded their specified temperature ('cos I don't use them). BTW - lead-acid batteries are usually charged at 14.4V (2.4V/cell). When full, they are usually reduced to a float voltage of 13.8V (2.3V/cell).