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House battery monitor
I am hoping someone can give me advice on monitoring batteries. Since i have the old needle battery meters am looking for something that is digital with an alarm to alert me when batteries are at min SOC. I was looking at the victron 702 which has an alarm for low SOC and voltage but in looking over my batteries i realized that the 24v battery bank has a 12v center tap for some of the house loads. Since the victron uses a shunt on the negative terminal to measure power going both ways im not sure if that would work. I doubt that i could monitor both voltages with the same meter. Would i be better off just getting a volt meter with an programable alarm instead of something that measures current? Is voltage a reliable indicator of actual SOC? Thanks in advace for any input. Jeff
Jeff and Lindsay
91 Marathon XL -
10 Replies
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You may want to look at the 712 with Bluetooth, it will do anything you want. No problem setting it up to monitor 24v and has a wide range of settings.
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Thanks Tom, I could get the 712 to have bluetooth. I may be misinterpreting how the shunt works. My issue is how to get it to keep track of power going in and out on the 12v center tap. I will not have an accurate charge state if its only monitoring the 24v connection.
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When I switched to LiFePo4 I thought long and hard about having a battery “fuel gauge”.
Bottom line however is in 31 years we never used auto-start and never had dead house batteries. The reason is simple. If we aren’t driving we are running the generator except for when we go to bed, as long as we do not need heat or A/C.
When we get up in the morning I start the generator and turn it off when we start driving again. So we either are running the generator enjoying all the luxury the coach has to offer or we are plugged into 50 amp shore power.
My wife and I never have to wonder if we can turn something on or if we are depleting the batteries. We drag that generator with us every where we go and we might as well enjoy it.
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Thank you Jon, I do exactly as you do. The generator runs for 3 hrs every morning. You are most likely correct that its fine with no monitor. I just feel I want to watch the batteries a little closer as they age. It also gives me something to obsess about. Your post gives me piece of mind while i work out a monitoring device.
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Jeff, I am assuming you have AGM batteries. I had Lifeline AGM for the house for at least 20 years and replaced them regardless of condition every 5 years. At the same time I changed my Delco 1150 chassis batteries, and my AGM Optima generator start battery.
Never had a battery problem.
Just to put this in perspective when we went to bed at night we turned the generator off. We had the refrigerator and bay refrigerator running, the aux compressor on, water pump on, power to the TV circuit, and 4 amps of phantom loads, such as CO alarm, clocks, various antenna boosters, coach alarm system, etc. By morning our house voltage based on an analog gauge was always around 24.8 give or take a tenth of a volt.
With Li the only change is I added a digital voltmeter which I didn;t need, and truth be told my analog gauge, my new “precision” digital gauge, my Outback inverter remotes and my Fluke RMS multitester all disagree by one or two tenths of a volt.
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Jeff, Isn’t your Marathon a 24 volt house and 24v inverters?
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Correct Tom. Ive been over thinking again. I can wire both (12v & 24v) of grounds through the shunt to monitor all current flow. The voltage doesnt matter on the ground.I didnt realize that the voltage is monitored with a separate positive wire. Im a moron. Thanks for the replies fellas.
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All good info Jon. I hear you and agree with what you are saying. I will continue to run my generator at the beginning and end of each day as you have always advocated. I just want something thats easier to read and has an alarm in case things get out of whack or i make a mistake. If i can get a few extra years out of my Lifelines its a big deal for me. Thanks, jeff
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Hi Jeff,
Battery voltage is a lousy way to assess lead acid battery SOC unless the batteries have been at rest neither charging or discharging for several hours. Even then it is at best a guess. 12vdc at rest is a lead acid battery that is at least 50% depleted and may well be further depleted than that. While running high amp draw AC appliances through an inverter you easily can see less than 11vdc on a volt meter even with fully charged batteries since one characteristic of any lead acid battery (AGM, SLA, flooded lead acid, etc.) is significant voltage sag under use.
As you have heard from others, unless you plan on regularly leaving the coach with AC appliances running, don’t worry much about your battery SOC, just run the generator for two to three hours in the morning and for an hour or two in the evening and you will be fine.
I split my house 12vdc battery bank into two banks, one lithium iron phosphate and the other 3 of my remaining AGMs. The inverter loads run only off the LiFePO4 batteries and the AGMs only see 12vdc loads like LED lights, motors for the pumps and Webasto, etc. First thing in the morning with shunt based battery monitors on both I see 30% or so SOC on the LiFePO4 bank and well over 60% on the AGM bank. An hour or two of generator running brings both back to 100% and I don’t need to run the generator in the evening. If you are interested you can read a five part article I wrote about how to do this in the last several issues of Bus Conversion Magazine.
Jerry
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Thanks Jerry. That’s a lot of good info. I will check out your articles if i can find them. Jeff
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