Need sanity check on heterogenous house bank configuration

[patrickstickler]

OK, so I have a new (old) boat that has two identical brand new 70AH maintenance free deep discharge house batteries hooked up in parallel and a traditional flooded starter battery. I have from my previous boat two nearly new 110AH AGM batteries that I'd like to add to the house bank.

Simply hooking the two 70AH and the two 110AH batteries together in parallel won't work because the 70AH batteries will drain faster and end up potentially being discharged too far and damaged before the AGMs discharge to the minimal safe level.

But I don't want to muck about with two completely separate house banks and constantly think about whether I'm using A or B today, etc. or have to remember if the switch is in the right position for use, charging, etc. etc.

So what I'm planning to do is to connect all four batteries together in parallel, but to use low voltage protectors on both of the 70AH batteries.



C.f. PriorityStart! - "Stop Dead Batteries" on Boats & Marine Equipment!

Thus, both 70AH batteries will reach the cutoff low voltage of 11.5 volts before the 110AH AGM batteries would, and at that point, will be automatically taken out of the bank, preventing over discharge and damage, and the 110AH AGMs will remain in the bank and will discharge to whatever level is demanded and allowed through normal usage and monitoring.

Then, when recharging, the 70AH batteries will automatically drop back into the bank when the charging voltage is sensed, and all batteries will be recharged together in parallel by the smart charger as usual, with each battery taking what it needs from the charger until all four batteries are fully charged.

Although the AGMs could be recharged at a higher amperage and voltage than the maintenance free batteries, the existing shore power smart charger and alternator are acceptable for both types of batteries in the house bank and so charging them all together should be fine. Likewise when later I add solar and or wind charging to the mix.

The entirely separate starter battery will be charged using an echo charger drawing from the house bank.

So... what am I missing or misunderstanding?

[mitiempo]

I would keep them as 2 separate banks. Mixing 2 types of battery (flooded and AGM) is not good, mixing batteries of different AH is not good and mixing old batteries with new isn't good. And by the way, if the cutoff is 11.5 volts that is about 90% down from a fully charged battery and they will have a very short life if drained that much more than a few times.

[btrayfors]

I agree with Brian, except that I'd get rid of the two 70AH "maintenance free" batteries....sell them or keep them in reserve or use them for starting if the start battery on your new old boat isn't great.

The hardest thing to do when you have "new" somethings which aren't just right is to bite the bullet and get rid of them, rather than try to accommodate what isn't right to begin with.

Then, if you need more than 220AH for the house bank, add another 110AH AGM or two. That would keep size and chemistry and age about the same, which is a very good idea.

RE: charging the start battery, yeah, an EchoCharge or DuoCharge could do that nicely.

Bill

[patrickstickler]

OK. Thanks for the input.

I think I'll keep the 70AH batteries as a separate "utility" house bank, on a second echo charger charged off the main bank, where it can serve as a backup to the primary AGM house bank, and I'll run the fridge, galley inverter, some ventilation fans, and a few other higher draw but less often used items off this "utility" bank, leaving the main AGM house bank for electronics, lighting, etc.

At some point in the future, when they are near the end of their life, I'll then look at expanding the AGM house bank to replace the second utility bank, and possibly even the separate windlass battery.

[btrayfors]

Sounds like a plan, but don't forget if you wait to add AGMs to the main house bank you will have batteries of different -- perhaps very different -- ages. Just a consideration to keep in mind.

Bill

[patrickstickler]

Originally Posted by btrayfors

Sounds like a plan, but don't forget if you wait to add AGMs to the main house bank you will have batteries of different -- perhaps very different -- ages. Just a consideration to keep in mind.

Bill

Certainly. Thanks.

[TQA]

Simply hooking the two 70AH and the two 110AH batteries together in parallel won't work because the 70AH batteries will drain faster and end up potentially being discharged too far and damaged before the AGMs discharge to the minimal safe level.

This actually does not happen because if one battery has a lower voltage then it gets charged back up from the other batteries until all the voltages are equal.

HOWEVER this is exactly why mixing batteries of different types and capacities is a really bad idea as they tend to try and charge up the weakest battery all the time. You can get away with it IMHO if you have a more or less continious charge going in eg wind or solar but it is best avoided if possible.

[chef2sail]

Refrigeration will be your largest draw off your batteries. It usuallu average anywhere from 3-5 amps per hour depending on the weather, and your insulation. A 70 aph battery really has only 35 amp hours before it is considered discharged. Your two in parralell will give you about 70 amp usable...nort really even enough to run the refrigeration 24 hours. My sugggestion is you Keep the AGM as you housebank and your workhorse. Use the other as a second bank only if you runn the main bank down. Echo charger from the main house bank to the starting battery.

For the future it is better to have one large house bank rather than 2 medium size ones you will switch from one to another. Also conside 6 volts. Twice as many batteries......same footprint in space.....twice the amp hours usually.

Dave

[patrickstickler]

Originally Posted by chef2sail

Refrigeration will be your largest draw off your batteries. It usuallu average anywhere from 3-5 amps per hour depending on the weather, and your insulation. A 70 aph battery really has only 35 amp hours before it is considered discharged. Your two in parralell will give you about 70 amp usable...nort really even enough to run the refrigeration 24 hours.

Fortunately the fridge only draws 300MaH average when cooled down (2-3 amps for an hour or so initially when first turned on) so the two 70AH batteries should do fine for now, and I don't consider the fridge essential equipment, so better to keep it off the main house bank driving the electronics. I minimize lighting consumption by using oil lamps alot, and plan to install LEDs in the nav lights. Heating will not require electricity except for a startup fan to aid draft until warmed up.

My sugggestion is you Keep the AGM as you housebank and your workhorse. Use the other as a second bank only if you runn the main bank down. Echo charger from the main house bank to the starting battery.

I think I mostly agree with your suggestions. Though if I have to have two entirely separate banks, I'd just as soon move the non-essential stuff off the house bank (which at 220total, 110usable is a little on the small side for longer trips) so that there is more available for electronics, nav lights, and other "modern essentials".

With the 60A shorepower charger I have, and two 15A echo chargers for the second bank and starter battery, and an occasional top up from the engine alternator, that should do me for up to a week at a time, I'm thinking, if I closely watch stuff like cabin lights, etc.

I'm also thinking to eventually install a second higher output alternator, though am weighing that cost with adding solar/wind. At the moment, solar is looking like it would be the most beneficial subsequent addition to the system, though a high output alternator may be more of a "sure thing" in these higher latitudes.

For the future it is better to have one large house bank rather than 2 medium size ones you will switch from one to another.

That was my motivation for this thread, to figure out how to build one single bank from the batteries I have.

Also conside 6 volts. Twice as many batteries......same footprint in space.....twice the amp hours usually.

Dave

That's my plan eventually, when I have to (and am flush enough) to replace all the batteries in the future.

[hellosailor]

You really don't want to mix the batteries into one bank. Those "starter protector" gizmos are designed only to cut out a single battery when it gets low, so there will be enough power left in it for starting. That's not going to prevent mismatched charging problems, it would be a total waste. Use one on your starting battery, if you need to, or KISS and get rid of it.

I'd use the two AGMs as a house bank, and combine the two "sealed" wet leads as a second bank. Either bank should be able to start your engine, so you can toss the starting battery, or, if it is similar to the two sealed ones, combine the three of them into one bank.

This rashly assumes that your AGMs use the same charging voltages as the wet lead batteries. If they don't--your system needs to be set for one or the other, and used for only one battery type.

If your AGMs and wet leads use the same profile, keeping them in two separate banks will "consume" them faster than one big bank, sure. But that's the way it has to be, they should not be combined into one bank. Once they are consumed, you replace them all with one bigger bank with matched batteries in it. That's really just the most effective way to do it, use what you have, and when it is used up, replace it with something better.