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post #1 of 136 Old 2 Weeks Ago Thread Starter
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Realistic expectations for my house batteries

My last boat was very minimalist, with only a 27ah house battery and only lighting loads, and most of the places we went we had access to shore power if needed, so I don't have a lot of experience with power management.

Our new boat has significantly more than that! We have a 550ah bank of flooded cell batteries providing power for a Webasto heater, refrigeration, electronics, auto helm, audio system, and even a 200w inverter powering a 22" led TV, (although I doubt I will use the TV much).

This weekend was our first time taking the boat "off grid", so I was watching battery voltages carefully to get some idea how long we could go without recharging. Unfortunately the boat is not yet equipped with an ammeter, so I don't know how much we were drawing. We started with fully charged batteries at 12.8v. Our main loads were the refrigeration and the heater. I have already upgraded all lighting to led so that is a negligible load. After 24hrs I saw the voltage dropping as low as 12.3v, but that was when both the heater and the fridge were running. When both units cycled off, voltage recovered to 12.5v

So when judging how discharged the bank is, should I be referencing the voltage under load, or at rest? My understanding is that you don't want to discharge more than 50% or about 12.2v. It seems to me my batteries were already getting low. I would have expected to be able to go longer than 24hrs without charging with my relatively large bank.

What should I expect from my batteries?

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Re: Realistic expectations for my house batteries

You really should expect nothing until you get a battery monitor and find out how much you are using on a daily basis. Voltage measurement should be at rest for guestimating SOC, but this is almost impossible to achieve on an actively-used boat.

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Re: Realistic expectations for my house batteries

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Re: Realistic expectations for my house batteries

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Originally Posted by colemj View Post
You really should expect nothing until you get a battery monitor and find out how much you are using on a daily basis. Voltage measurement should be at rest for guestimating SOC, but this is almost impossible to achieve on an actively-used boat.
Yeah a voltage monitor is on my to-do list once I do my homework, and decide which one to get. Apparently all I need is a shunt for my panel to display amp draw. Don't ask me why that would not be installed from the factory...seems like it should be standard equipment on such a boat! Regardless, it doesnt give the whole picture because not all loads go through the panel. The heater, and power winch are not on breakers from the panel.

If resting state is the best way to check SOC, it's not hard to shut off all major loads once in a while to check it.

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Re: Realistic expectations for my house batteries

Hey,

it's been my experience from my RV days that a forced air heater consumed a lot of power. If it was 50 or below the house battery would be dead by morning. If it was 60 and above I didn't use the heater and the battery would last 2-3 days. If you will be using the heater often then you better find out how much power it uses. If you won't be using the heater then your will use a lot less power.

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Re: Realistic expectations for my house batteries

Welcome to the fight. Power management and tracking can be quite consuming, especially on a power hungry boat. Triple if your OCD about things like this.

A shunt and battery monitor are minimum ante into the game. You need to know how much draw you typically have, but you also need to know how much charge is going back in. Knowing how much charge is going back in may even be more important, because it's the only way to really know if you've properly topped up a flooded acid bank. You need to see how much acceptance amperage there is, not just count amps. Topping up increases longevity (is saves money) and maximizes capacity.

You are right, the rule of thumb is that one shouldn't use more than 50% of lead acid capacity, but capacity lowers as batteries age. It's hard to know what capacity really is after a few years. On the other hand, there is no law about that. It's simply considered the level that balances the number of useful charge cycles in the battery's lifetime against useable capacity. You can discharge to a lower State of Charge SOC, getting more useable capacity, but the batteries will die sooner. Vice versa, as well. Be light on their usage and they last longer, but of course, are less useful.

Balmar sells a Smartguage that is supposed to be able to determine SOC in a percentage, after it automatically "learns" you bank over a few cycles. I have one. It has allowed me to identify some real flaws in the OEM setup, which I've fixed. It also caused me to identify that I wasn't maintaining full charge, due to poor charge settings. I'm still not fully convinced it works, but I have to give it credit for causing me to debug. This will be my first season with both new wiring and charge settings, so I'll know over the next month or so, if I think it's SOC is accurate. Some swear by it.

Finally, 550 ah may not be enough for a power hungry boat. We only have 440ah and there is not way I can make it 24 hours with everything on...... ie cruising. I have to run the generator at some point. When I run this bank to it's demise, I'm upgrading to lithium. Twice the capacity in the same footprint and much more efficient charge profiles, without the need to top up to prevent decay.


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Re: Realistic expectations for my house batteries

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Originally Posted by SchockT View Post
Yeah a voltage monitor is on my to-do list once I do my homework, and decide which one to get. Apparently all I need is a shunt for my panel to display amp draw. Don't ask me why that would not be installed from the factory...seems like it should be standard equipment on such a boat! Regardless, it doesnt give the whole picture because not all loads go through the panel. The heater, and power winch are not on breakers from the panel.

If resting state is the best way to check SOC, it's not hard to shut off all major loads once in a while to check it.

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Those won't be a problem to measure because a battery monitor shunt is wired so that all loads go through it, not just the panel - panel, alternator, all loads.

You want more than a voltage monitor. Get one that monitors voltage, amps, cumulative Ah.

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Re: Realistic expectations for my house batteries

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Originally Posted by Minnewaska View Post
Twice the capacity in the same footprint
Twice the usable capacity in 1/3 of the footprint and 1/4 of the weight.

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Re: Realistic expectations for my house batteries

Using an electric heater when boat is off grid is way too optimistic. Put on some extra clothes and a hat. You will be pushing it with just your refrigerator running. You definitely need an ammeter to figure out what your equipment draws. When we lost generator and engine off the Honduras Banks, our 200 W solar panels were barely adequate for keeping navigation equipment running. It was not enough to keep our running lights on all night. We would only turn them on when we spotted another ship. Conserving power is very important.

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Use a clamp multi meter to get accurate draw...by item.
Then learn how long they run...24/24..4/24..etc
Quick to figure out
I have a bmv-700 but only use it to display voltage.
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