adding a battery

[Aquah0lic]

I know this can't be that hard but I can't wrap my head around it.

I've got a 2 100A battery system with a 1/both/2 rotary switch. I want to buy a new 100A battery and put it in an adjacent compartment to be used as my starting battery and make my current 2 my house bank.

How would my wiring have to be changed to accomodate this plan?

[limmer]

just add the 2nd battery to one of the bateries in parallel (pos to pos and neg to neg). I use the 1 on my rotary as my start so no one forgets 1 means 1 battery. and 2 means 2. I actually had to put start on the rotary switch in stead of the 1 just to make even more dummy proof.............didn't marry her for her brains.

[Aquah0lic]

Beautiful. I knew it was easier than I was thinking. I hadn't even considered doing it this way. Thanks for the tip.

[chala]

batteries in parallel? I know a lot of people will desagree with me but I will never wire batteries in parallel without at least using one bloking diode to prevent self discharge. There is commercial paralleling information easly available from BEP marine and 4WD equipment dealers. The difficulty is which system to install.

[anotherT34C]

I'll second the above. A new battery probably shouldn't be wired in parallel with an old battery. The old batt will possibly self-discharge and draw from the new, reducing its life.

I'd add a blocking diode between them. Or maybe gather the two old batts into the parallel circuit, and make the new one the lone starter. Or both.

[Pblais]

If you add a second 1/2/both it would allow you to start the engine from it so that you might on a rare occasion use the whole works to crank the engine. It may seem unlikely but I needed it just one time. I would not have started the engine otherwise. It was the last day of a 18 day trip and the last time I ever started the engine using any of the batteries. 6 golf cart batteries died on the very last day of the trip. I bought the battery monitor shortly there after.

I would also agree with AnotherT34C. You want a battery bank to be of similar parentage and date of birth. The the problem is one cell with a lower than normal voltage level will drag down the entire bank. It's not much of a drip but it signals the regulator to send a little more power. This has the effect of overcharging the rest of the cells in the bank causing them to self destruct sooner until all cells reach an equal level of destruction. It happens faster than you think.

It was (and is) a common problem with flood cell batteries. The idea is to use the 1/2/both switch to "switch out" on or more batteries that you notice going bad (having to add way more water to one cell vs another). Culling the herd to save the flock. Switches are easier than stretching the battery cables that don't reach. At the time it happens it won't be a good time.

If you have three unequal batteries of dissimilar parentage or birthday then I suppose go with the two closest in age and make them sisters and use the remaining to start the engine. If all three batteries are way different then it's not going to matter and you'll just wear them out unequally (meaning early). I just would never replace two without replacing them all or at least making the best of the three the new starting battery and making the two new ones the new bank. If right now the house battery is at all old I would go for two new ones and start your trip on high ground (battery speaking).

Wiring 2 - 1/2/both switches could drive you nuts. It is something you really really don't want to screw up. You need to draw a picture as well as measure the wires. The wires should all be nice and neat with heat shrink tubing and all pro crimped to as short a length as possible. It takes a while to do this properly.

[Aquah0lic]

Originally Posted by anotherT34C

I'll second the above. A new battery probably shouldn't be wired in parallel with an old battery. The old batt will possibly self-discharge and draw from the new, reducing its life.

I'd add a blocking diode between them. Or maybe gather the two old batts into the parallel circuit, and make the new one the lone starter. Or both.

One of the batteries I just bought 2 months ago and the other was approx 1 yr old when I got the boat. I want to tie the new ones together and use the old one for starting.

However, I usually keep the switch on both. In that case it seems like the old battery could still pull down the new ones. Is that right?

If I leave the switch on 1 all night, in the morning should I switch to 2 to start then switch to both or can I switch to both and start. What's the real difference?

[Charlie]

With the age difference in your batteries I don't think that you will have a problem. The goal is to make sure that you have at least one good battery to start the engine with. In order to do that you will need to not keep the switch on both when anchored. Keep the switch on the two battery bank (2) to run your house needs. You will need to consider charging though. When the time comes to start the boat you can put it on both and use the power from all three batteries. If you leave the switch on the starting bank (1) you will not get any charge on your house batteries. On some alternators and switches there is a problem with moving the switch while the engine is running. I think that an electronic part (diode?) in the charging system can get blown rendering the alternator in need of repair. The best explanation I have ever read is in the Caulder book. It is a great resource for any boater. In one repair you can make back the cost of the book.

[chala]

Aquaholic
I come from a school of thought where paralleling of battery is a no no even with new batteries. Laboratory test have shown that the paralleling of two new “identical” batteries will discharge given time. The description of the process given by the scientists was that each battery was charging each other until flat. The reason for the process was that at that time it was not possible to build two “identical” batteries. Recommendation given for increasing capacity was to increase cells capacity. It is well know in the steel boat building industry that the best practice for minimising “corrosion” would be to build with identical steel composition originating from the same batch. Recently in the UK a coroner attributed a loss of life in a boating accident to the internal short-circuiting of a battery in a parallel configuration.” Someone could check on that”, leaving the boat without electricity. Apparently the boat was new. Note that a blocking diode in this situation will only be 50 % effective in preventing that sort of accident. As you can see at Piranha - Dual Battery Isolator Summary. they have 3 systems, diode, relay and DSE. Piranhas used to produce better information’s. Clusters - BEP Marine give better information and they are good people to deal with. Some of the Bep switches that I bought fell apart, the Australian vendor would not replace them. I wrote to Bep in NZ.
Soon after I received a phone call from them and later a box of replacement with extra switches, something not done to often these days. On my boat I have a 12/24V system made of 5 batteries and a battery management system of my own design that I hope to describe in the future in another thread.

[Christian Van H]

Hi Aqua. If you do want to keep a second battery isolated, but fully charged, consider using an "EchoCharger" Echo Charge Battery Charger from XANTREX to isolate and keep the starting battery topped up. It senses when a charging voltage is applied to the house bank, and feeds the starting battery current automatically. Hope your havin' fun, Chris

[DeepFrz]

Laboratory test have shown that the paralleling of two new “identical” batteries will discharge given time.

Could you please cite a scientific paper that backs up this assertion. Personally, given the posts by Rick on this forum (addressing the paralleling of batteries) and given the commonality of paralleling batteries without incident I think this is misinformation. Paralleling of cells is exactly what you are doing when you parallel batteries. The only difference is you have fewer external connections when you parallel batteries.

[Sam Plan B]

Concerning flat batteries, I have considered buying a small gas powered generator. The idea is, store the generator until needed and then bring it up on deck. Plug in the shore power cable and start it up. The generator would provide power for the onboard battery charger and so charge up the battery bank allowing one to start the engine. Comments please.

[Pblais]

Could you please cite a scientific paper that backs up this assertion. Personally, given the posts by Rick on this forum (addressing the paralleling of batteries) and given the commonality of paralleling batteries without incident I think this is misinformation.

It's only sort of true. a common 12 volt battery is 6 - 2 volt cells in parallel. With flood batteries it was / is very common for only one cell to go bad. This weak cell would make the whole battery appear discharged thus fooling the smart charger into sending more current. This would cook the remaining 5 good cells. The ability to switch out a bad battery from a multi battery bank is a good idea if using 6 or 12 volt flood batteries.

So yes one bad cell can trash the whole battery and one cell gone bad in a whole bank could do the same. The part that is wrong is that you can escape paralleling batteries cells - you can't.

Concerning flat batteries, I have considered buying a small gas powered generator.

Many subscribe to using a small Honda gasoline power generator to charge batteries. It works to a point. You have to carry a lot of gasoline and the output means you really can never get close to charging the bank up that far. It does mean if you want to use a corded electric drill or other gadgets you now can get AC power. It works better for that application than charging batteries. The last friends I know that did it said they could go 3 weeks before they needed to equalize the bank. They used giant industrial flood battery with very very thick plates. You can never hope to equalize a battery bank with a small portable gasoline generator. On larger boats the diesel genset is the way to make more power often. It is far more efficient and cost effective when you really need that much power.

[DeepFrz]

Paul, I understand the bit about the bad cell, but I don't think that is what he was saying. My understanding was that he was saying don't parallel batteries cause they will discharge each other.

[Pblais]

My understanding was that he was saying don't parallel batteries cause they will discharge each other.

What he said:

Laboratory test have shown that the paralleling of two new “identical” batteries will discharge given time.

I don't need laboratory tests to prove if you leave a battery siting long enough it will discharge even if not paralleled. We obviously don't understand him. We will have to wait to hear about his 5 battery 12/24 volt system that he designed himself.