How LOW can you go? (Voltages that is...)

I have a 660A/h Firefly Carbon foam house bank on our Nordic Tug 37. We monitor State of Charge (SOC) with a Victron BMV701 shunt monitor.

I have been considering switching to a LiPO4 bank for the steady 12.8V they would provide and the opportunity to reduce our generator times with their much higher charge acceptance; but, in reality, our 5 year old Carbon foam batteries are still quite functional and the cost of switching to LiPO4 is not insignificant.

What bugs me with our current House Bank is that as I get down to ~ 50-55% SOC, the voltage starts to approach 12.1V. This is always more of an issue in the wintertime as we run a Wallas heater 24/7, and have higher draws. There is also very little solar charging to supplement the house bank. I do not feel too concerned about drawing the house bank down to even 40% SOC as the Fireflies are pretty tolerant to lower SOC states and also tolerate partial SOC conditions quite well but I don't like the associated lower voltages that come along with this.

I was under the impression that I should NEVER let the house bank voltage drop below 12.1V while running our various appliances, instruments etc. I’m not sure where I got this idea but I’ve always been quite paranoid as we approach these voltages.

I have been questioning this assumption though and now read that the Wallas heater (once it has started up) should be operational down to 11.0V and the NMEA 2000 network and instruments are supposed to be good to below 10.0V. The inverter can run with house bank voltages as low as 10.0 VDC.

I don’t know what the DC fridge & freezer need for proper functioning…

So, maybe I shouldn’t worry if we drop below 12.0V? (Note that the Start Battery is isolated behind an ACR relay and the generator start battery is completely independent of the House Bank.)

If allowing the house bank to get down to ~ 11.8 V is OK, we would then have quite a bit more reserve and quite adequate battery performance and I don't have to wake up in the morning feeling anxious as we flirt with 12.0V... If 12.8V is OK, I could be quite content to hold off on switching to LiPO4 for a few more years.

So... my question:

Do you have a voltage that you would consider to be the lowest safe value before you want to recharge? What value is too low for proper functioning of instruments / appliances etc.? Please explain your reason(s) for your voltage threshold.

Thanks for your input.

-evan

I worry about indicated SoC, not voltage on my bank. Voltage will vary widely based on the load the batteries are under with a lead-acid bank. A really big load on the inverter can pull my bank down to 11.8 or for a few minutes even at 80% SoC. Voltage comes back up once the load is reduced.



With older batteries, I'd pay attention to see if the voltage is getting lower and lower at a given SoC and load, as that will indicate that you're seeing noticeable capacity loss from the batteries. You could do a 20 hour load test to determine what their real capacity is at this point if you're concerned.

rslifkin said:

I worry about indicated SoC, not voltage on my bank. Voltage will vary widely based on the load the batteries are under with a lead-acid bank. A really big load on the inverter can pull my bank down to 11.8 or for a few minutes even at 80% SoC. Voltage comes back up once the load is reduced.



With older batteries, I'd pay attention to see if the voltage is getting lower and lower at a given SoC and load, as that will indicate that you're seeing noticeable capacity loss from the batteries. You could do a 20 hour load test to determine what their real capacity is at this point if you're concerned.

Click to expand...

I understand the concerns around deteriorating capacity. I think they are actually pretty good. Firefly recommends occasionally taking them down to 10.5V and then hitting them as hard as you can with as much charging as you can muster. I just drew them down to 23% SOC for a battery conditioning sequence on the weekend (they were at 11.42V & -527 A/h before I starting re-charging). I can give them 260 - 280 amps of charging with the generator chargers, alternator and solar panels.

Even at 50% SOC, the bank seems to have very little voltage drop with higher loads (e.g. microwave use) but if we drop to 11.8V in that situation, should I be concerned?

Should I worry about damage to the instruments / DC appliances at 11.8V or even 11.5V?

On my FLA bank at 50% per the manufacturer the voltage will be 11.6V with normal loads applied.

Your Victrom BMV tells you all you need to know. Voltage readings of batteries in use are pretty much useless. Batteries need to be unused and at rest for 24 hours for voltage to mean anything. Your BMV is a coulomb counter - electrons in, electrons out.

catalinajack said:

Your Victrom BMV tells you all you need to know. Voltage readings of batteries in use are pretty much useless. Batteries need to be unused and at rest for 24 hours for voltage to mean anything. Your BMV is a coulomb counter - electrons in, electrons out.

Click to expand...

Yes. I’m not using voltage as a measure of SOC.

I want to know what the lowest safe operating voltage should be…

ksanders said:

On my FLA bank at 50% per the manufacturer the voltage will be 11.6V with normal loads applied.

Click to expand...

That’s interesting. Thank you.

Are you happy running your system at that 11.6V?

How much of your loads that aren't happy below 12 VDC can be run through the inverter. Most of my appliances including refrigerator, freezer, microwave, and a few other things will or do run on 120 VAC. The inverter doesn't hiccup until 10 volts. The answer might be to switch to 120 VAC when the bank drops below 12 VDC.

Ted

O C Diver said:

How much of your loads that aren't happy below 12 VDC can be run through the inverter. Most of my appliances including refrigerator, freezer, microwave, and a few other things will or do run on 120 VAC. The inverter doesn't hiccup until 10 volts. The answer might be to switch to 120 VAC when the bank drops below 12 VDC.

Ted

Click to expand...

That's a good idea particularly if the fridge / freezer are more sensitive to lower DC voltages. I have more homework to do there... are they actually more sensitive? Our inverter (Magnum 12/2800) draws ~3 Amps at idle so best left off unless actually needed for AC only appliances.

Most of the heavy load DC draws would only be in action when the engine is running. (Bow / stern thrusters, windlass etc.)

eheffa said:

That’s interesting. Thank you.

Are you happy running your system at that 11.6V?

Click to expand...

Yes, why not????

DC devices are typically rated between 10.5 and 14 volts. Anything in between that is perfectly fine.

It's not like I run at 11.6 all day long, or even much at all. If I go to bed early my 140AH bank will be at around 50% by 7:00 AM. I get up and start the generator, and put on coffee.

ksanders said:

Yes, why not????

DC devices are typically rated between 10.5 and 14 volts. Anything in between that is perfectly fine.

It's not like I run at 11.6 all day long, or even much at all. If I go to bed early my 140AH bank will be at around 50% by 7:00 AM. I get up and start the generator, and put on coffee.

Click to expand...

Thank you for your comments ksanders. I appreciate that devices running on nominal 12 VDC devices have a fairly wide range of operating voltages but some devices are more sensitive than others. (e.g. my Furuno TZT215 Chart plotter is very sensitive to changes in voltage and will reboot when I start the engine.)

I asked this question to see if people are aware of any risks to their assorted 12 VDC instruments or appliances if they allow their house bank to drop into the 11.6 - 11.9 VDC range?

Since the Furuno has shown an inability to tolerate the lower voltages, I just bought a Victron DC-DC converter to power the chart plotter (and the NMEA 2000 network) hoping that they can be protected from low voltage 'brownouts'. I'm not sure that I have any other devices that need this kind of extra protection from low voltages...


-Evan

eheffa said:

Thank you for your comments ksanders. I appreciate that devices running on nominal 12 VDC devices have a fairly wide range of operating voltages but some devices are more sensitive than others. (e.g. my Furuno TZT215 Chart plotter is very sensitive to changes in voltage and will reboot when I start the engine.)

I asked this question to see if people are aware of any risks to their assorted 12 VDC instruments or appliances if they allow their house bank to drop into the 11.6 - 11.9 VDC range?

Since the Furuno has shown an inability to tolerate the lower voltages, I just bought a Victron DC-DC converter to power the chart plotter (and the NMEA 2000 network) hoping that they can be protected from low voltage 'brownouts'. I'm not sure that I have any other devices that need this kind of extra protection from low voltages...


-Evan

Click to expand...

Engine start can often cause voltages to dip down to 10 volts or so, which is below what many electronics will handle. Many get unhappy around 10.5 volts. 11.5 wouldn't worry me on most stuff, although things like bilge pumps will lose some capacity at that point.

The engine start reboot is why I really dislike starting engines from the house bank.

ksanders said:

Yes, why not????

DC devices are typically rated between 10.5 and 14 volts. Anything in between that is perfectly fine.

It's not like I run at 11.6 all day long, or even much at all. If I go to bed early my 140AH bank will be at around 50% by 7:00 AM. I get up and start the generator, and put on coffee.

Click to expand...

I am thinking you have a 1400 Ah bank and using 700 overnight. Or is that in 24 hours needing generator once a day, for how long at what charge rate.

SteveK said:

I am thinking you have a 1400 Ah bank and using 700 overnight. Or is that in 24 hours needing generator once a day, for how long at what charge rate.

Click to expand...

We have a 660A/h Firefly (carbon foam) house bank.

In wintertime with little solar panel contribution and the heater running 24/7 we average 15-17 Amps per hour (~360 A/h per 24 h) so we are dipping into the ~50% SOC range. I’m not concerned about that but rather as we approach those SOC values, we see voltages hovering around 12.0V. Should I be concerned about that ? Thus the reason for my post.

The generator run will be ~ 2-2.5 hrs per day to achieve a return to ~ 90+ SOC. (We have a 9.5KW Onan generator with 2 AC chargers capable of ~160A. Add the 120A engine alternator & we get to 280A in theory anyways. ). In spring through fall, the 550W of solar also adds a significant boost.

If at anchor a few days, this cycle repeats for as many days as we are on the hook.

eheffa. Your use seems normal. The recharge time suggests the genny is powering approx 80amp charger.
That is the area I am looking into now, recharge time & how many amps can the FLA accept in shortest time.

Three times in the last eight years while in our winter slip I have inadvertently run our 440 A/h Firefly bank down below 11.4 volts. We also run a Wallas heater during the winter, 24/7. But on two of those occasions what made me realize I hadn't turned the battery charger back on was the water pump came on and sounded a bit sluggish.

Otherwise, all 12V equipment seemed to operate normally at low voltages. And the batteries bounced back just fine.

Anecdotal, but that's my real life experience with the same issue...

SteveK said:

I am thinking you have a 1400 Ah bank and using 700 overnight. Or is that in 24 hours needing generator once a day, for how long at what charge rate.

Click to expand...

I have four L16HC FLA batteries 420 amp hours each, or 840 for the bank.
Overnight I use about 50% of that based on my Victron SOC meter.

I find that I run the generator in the morning to recharge, and I also like to run the watermaker, clothes washer, and the stove for breakfast during that time. My recharge time is about 2 hours. for charging I have two Victron inverter/chargers in parallel mode and the charge current self limits at 200A (into the batteries) based on my 35A max inout AC current setting.

The solar on the pilothouse keeps up pretty well during the day and in the evening we run the generator long enough for cooking dinner and this provides enough charge to bring the batteries up into the 90% range before bed time.

As a side note i installed a input to the Victron chargers to limit charge current to 100A if the manin engines are running. That way we do not over current the house batteries if you get underway early and have the generator on.

SteveK said:

eheffa. Your use seems normal. The recharge time suggests the genny is powering approx 80amp charger.
That is the area I am looking into now, recharge time & how many amps can the FLA accept in shortest time.

Click to expand...

Hi Steve,

Even when running the engine and generator at the same time we can see 260 Amps of charging for a short while but this starts to taper off in an almost 2:1 ratio. The charge acceptance falls off as the SOC climbs so by the time we are at a 200 A/h deficit the charge acceptance will only be around 100 A. (This is despite having the chargers in a CC/CV protocol and the Balmar alternator setup for a longer duration bulk charging...).

I believe this charge acceptance taper is simply the nature of any lead acid battery bank and a major reason to switch to LiPO4 when it comes time to replace the house bank.

I have considered adding a third AC charger to augment the charging capacity but I'm not sure how much difference that would make to the overall charging times.

-e

Moonfish said:

Three times in the last eight years while in our winter slip I have inadvertently run our 440 A/h Firefly bank down below 11.4 volts. We also run a Wallas heater during the winter, 24/7. But on two of those occasions what made me realize I hadn't turned the battery charger back on was the water pump came on and sounded a bit sluggish.

Otherwise, all 12V equipment seemed to operate normally at low voltages. And the batteries bounced back just fine.

Anecdotal, but that's my real life experience with the same issue...

Click to expand...

Hi Darren, I appreciate your observations here.

What sort of voltages do you see at 50% SOC?

-e

I have presently 300 usable amps of DC in my battery house bank, and 600 watts of solar panels. Given my present setup I can go 24/7 indefinitely.

For example, my Marina gives you the option to lease a slip with or without power. I chose to go without electricity and saved several hundred dollars. We did the entire 2022 season on the boat and never needed AC power. There was only twice in the entire season that I ever ran my generator for additional AC when I needed to run a large power device such as a shop-vac, and that was for about 20 minutes at a time each time.

This winter I ordered a much larger DC to AC inverter. We did okay last summer, but needed to run the generator if we wanted to run a microwave or shop-vac. So I am making plans to go even larger on my battery bank and solar array. By the sprink i will have 800A of usable battery power. I have the room so why not. I'm hoping with the larger Bank, I will not have to run to generator at all in the 2023 season.

Max1 said:

I have presently 300 usable amps of DC in my battery house bank, and 600 watts of solar panels. Given my present setup I can go 24/7 indefinitely.

For example, my Marina gives you the option to lease a slip with or without power. I chose to go without electricity and saved several hundred dollars. We did the entire 2022 season on the boat and never needed AC power. There was only twice in the entire season that I ever ran my generator for additional AC when I needed to run a large power device such as a shop-vac, and that was for about 20 minutes at a time each time.

This winter I ordered a much larger DC to AC inverter. We did okay last summer, but needed to run the generator if we wanted to run a microwave or shop-vac. So I am making plans to go even larger on my battery bank and solar array. By the sprink i will have 800A of usable battery power. I have the room so why not. I'm hoping with the larger Bank, I will not have to run to generator at all in the 2023 season.

Click to expand...

Interesting strategy Max.

I take it you are using the boat mostly during the summer months? Do you really get enough sunshine in the winter in Toronto to use the boat without shore power or generator use? (We have 550W of solar but there are grey days when we don't get anything more than 5 A of charging at any one time during the day.)

My mechanic tells me that I should make sure to run our generator regularly as having it sitting idle for a long time is death to a diesel; but, I wouldn't mind making the generator runs shorter. Not using the generator more than once or twice a year may not be good for its longevity...?

A different point of view: I have way more faith in state of charge derived from battery voltage data published by reputable battery vendors than by algorithms in chargers or monitors. And while I agree most DC loads are tolerant of lower voltages, that is not true for danfoss/Secop compressors found in most DC refrigeration. Finally, my own experience over almost two decades of active cruising anchoring around 100 to 150 days a season is that battery life for lead acid batteries is greatly reduced when they are routinely discharged below 12VDC.

Jc180 said:

A different point of view: I have way more faith in state of charge derived from battery voltage data published by reputable battery vendors than by algorithms in chargers or monitors. And while I agree most DC loads are tolerant of lower voltages, that is not true for danfoss/Secop compressors found in most DC refrigeration. Finally, my own experience over almost two decades of active cruising anchoring around 100 to 150 days a season is that battery life for lead acid batteries is greatly reduced when they are routinely discharged below 12VDC.

Click to expand...

I appreciate your comment on refrigerator compressors.

I'm not sure that I would agree with you regarding how to monitor SOC. Compared to using a proper shunt and monitoring actual current utilization, a pure voltage reading and referencing a chart is cannot be considered to be a reliable indicator of SOC.

I agree with you that in general, discharging below 50% SOC will degrade the batteries earlier than only drawing down to say 75% but in the case of Carbon Foam, they are a little more tolerant of deeper discharges.

I can report that since doing a conditioning cycle for my Carbon Foam bank (i.e. a deep discharge to ~ 11.3V and following up with an aggressive re-charge), the voltages are staying higher for any given state of charge. This is further evidence that voltage values are not a good indicator of SOC.

eheffa said:

Interesting strategy Max.

I take it you are using the boat mostly during the summer months? Do you really get enough sunshine in the winter in Toronto to use the boat without shore power or generator use? (We have 550W of solar but there are grey days when we don't get anything more than 5 A of charging at any one time during the day.)

Click to expand...

Where we are, winters get pretty cold and almost all boats come out for the winter except the live-aboards that use bubblers to keep the water around the boat from freezing. Right now in the dead of winter, our boat is drydocked and wrapped for winter hibernation..... So I guess my caveat is: "We have enough power during the summer months"

eheffa said:

My mechanic tells me that I should make sure to run our generator regularly as having it sitting idle for a long time is death to a diesel; but, I wouldn't mind making the generator runs shorter. Not using the generator more than once or twice a year may not be good for its longevity...?

Click to expand...

That is probably a good idea. I have a small generator and rarely used it last year. Running it every once in a while makes sense.

Jc180 said:

A different point of view: I have way more faith in state of charge derived from battery voltage data published by reputable battery vendors than by algorithms in chargers or monitors. And while I agree most DC loads are tolerant of lower voltages, that is not true for danfoss/Secop compressors found in most DC refrigeration. Finally, my own experience over almost two decades of active cruising anchoring around 100 to 150 days a season is that battery life for lead acid batteries is greatly reduced when they are routinely discharged below 12VDC.

Click to expand...

"battery life for lead acid batteries is greatly reduced when they are routinely discharged below 12VDC". No big surprise. You will find that in any battery manufacturer's data sheet.

eheffa said:

I appreciate your comment on refrigerator compressors.

I'm not sure that I would agree with you regarding how to monitor SOC. Compared to using a proper shunt and monitoring actual current utilization, a pure voltage reading and referencing a chart is cannot be considered to be a reliable indicator of SOC.

I agree with you that in general, discharging below 50% SOC will degrade the batteries earlier than only drawing down to say 75% but in the case of Carbon Foam, they are a little more tolerant of deeper discharges.

I can report that since doing a conditioning cycle for my Carbon Foam bank (i.e. a deep discharge to ~ 11.3V and following up with an aggressive re-charge), the voltages are staying higher for any given state of charge. This is further evidence that voltage values are not a good indicator of SOC.

Click to expand...

Thanks for the discussion. Not to be argumentative, but could also make the case that voltage values are the point of reference and therefore say that SOC algorithm is not accurate. Once again, all reputable lead acid battery vendors publish SOC values as a function of voltage. In your example, actual battery state of charge likely higher than the algorithm calculation. Think we agree though on the important point and that is routine deep discharge cycles shorten battery life.

Jc180 said:

Thanks for the discussion. Not to be argumentative, but could also make the case that voltage values are the point of reference and therefore say that SOC algorithm is not accurate. Once again, all reputable lead acid battery vendors publish SOC values as a function of voltage. In your example, actual battery state of charge likely higher than the algorithm calculation. Think we agree though on the important point and that is routine deep discharge cycles shorten battery life.

Click to expand...

Those values assume no load on the batteries though. Because of voltage sag under load it's hard to determine SoC from voltage on a boat. I routinely see a 0.2 volt difference in battery voltage as things like the fridge cycle on and off.

Jc180 said:

Thanks for the discussion. Not to be argumentative, but could also make the case that voltage values are the point of reference and therefore say that SOC algorithm is not accurate. Once again, all reputable lead acid battery vendors publish SOC values as a function of voltage. In your example, actual battery state of charge likely higher than the algorithm calculation. Think we agree though on the important point and that is routine deep discharge cycles shorten battery life.

Click to expand...

Said another way, improved battery SOC response to conditioning cycle more accurately represented by the resultant voltage than the simplistic algorithm of using load and charging amps over time and comparing to a set amp hour value. In other words, the SOC performance improvement due to internal battery chemistry change after conditioning not recognized by algorithm.

And works the other way as battery ages and performance drops off. In such case, battery voltage would indicate lower SOC than algorithm.

Most of your electronics are find at lower voltages, but that doesn't mean it's great for them, especially if smaller wires. (lower voltage = more amps/heat for same power/watts).

You can look at low voltage cutoff on your equipment. Most will be 10-10.5VDC. Above that should be ok.

The only FF voltage/SOC chart I've ever seen was from 2016. It claims these values at C/10 discharge rate.

13.35 100%
12.78 90%
12.64 80%
12.50 70%
12.34 60%
12.19 50%
12.02 40%
11.84 30%
11.63 20%
11.49 15%
11.33 10%
11.10 5%
10.50 0%

Personally I find these a little (0.1+V) optimistic even when new. We have six L15% FF Oasis 4v batteries in 3S2P arrangement. Soon to be replaced with LiFePO4 partly due to voltage sag. The wife likes to use start very large electric loads first thing in the morning and the load(s) would pull voltage down to 11.5 ish triggering my low voltage alarm setting.

Something else mentioned in this thread is using DC-DC to keep voltage steady. Keep in mind that those waste 11-12% during the process according to specs. I've never measured the actual waste. It could be higher.

Voltage can be an excellent measure of the SOC. It is best coupled with an AH counter. We currently use a BMV-712. As demonstrated in this thread, when the SOC meter reads 50% but the voltage (under similar load) is lower than it used to be, that is the SOC meter reading incorrectly, not the voltage meter.

Chris
SV Cosmos

rslifkin said:

Those values assume no load on the batteries though. Because of voltage sag under load it's hard to determine SoC from voltage on a boat. I routinely see a 0.2 volt difference in battery voltage as things like the fridge cycle on and off.

Click to expand...

The manufacture of my house bank crown publishes a battery voltage reference with load and for example 11.6 V is 50% discharged.