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

[SteveK]

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

11.6v is 50% while under load, correct.

 

[Insequent]

The OP has not indicated what DC fridge he is using, but as noted in post #22 Danfoss compressors are sensitive to voltage. Indeed, you have to by careful about length of cable runs and use appropriately heavy gauge wiring.

Below is an extract from the Waeco manual. The fridge can be programmed to turn off at 3 different battery voltages, a feature intended for car users with only one battery. Note that these voltages are at the fridge itself ie after voltage at the battery and voltage drop along the wiring. For normal operation with a seperate house battery bank to the start batteries it is recommended to use "low". Which is OK, except that the cooling performance of the fridge is very poor when the voltages get down a bit. My Waeco's would not keep stuff frozen when my house bank was due for replacement. Unfortunately I did not measure voltage at the fridge when this was happening .....



6.4 Using the battery monitor.
The device is equipped with a multi-level battery monitor that protects
your vehicle battery against excessive discharging when the device is
connected to the on-board 12 V supply.
If the cooler is operated when the vehicle ignition is switched off, the
cooler switches off automatically as soon as the supply voltage falls
below a set level. The cooler will switch back on once the battery has
been recharged to the restart voltage level.
Caution – Danger of damage!
When switched off by the battery monitor, the battery will no longer be
fully charged. Avoid starting repeatedly or operating current consumers
without longer charging phases. Ensure that the battery is recharged.
In “HIGH” mode, the battery monitor responds faster than the levels
“LOW” and “MED” (see the following table).
Battery monitor mode LOW MED HIGH
Switch-off voltage at 12 V 10.1 V 11.4 V 11.8 V
Restart voltage at 12 V 11.1 V 12.2 V 12.6 V

 

[backinblue]

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

My instruments will issue a low voltage warning then eventually shut down. Found this the hard way on the first trip on new to me boat. My water pump is surprisingly sensitive to low voltage. It will cycle on and off if the voltage is low. My guess is the pressure sensor is not accurate below a certain voltage. This often happens at the worst time in the early morning hours. I'm adding some solar this year to keep the batteries topped up at anchor to see if that helps. I do run the generator, but not always long enough for a full re-charge.

 

[ksanders]

11.6v is 50% while under load, correct.

Thanks!!! This is something folks just do not understand.

It is perfectly normal for a FLA bank to be below 12V while in operation.

At 50% discharge when I get up in the morning my Vaccuflush heads work. The water pump works. The electronics are still on. The inverter is working properly.

Pretty much every piece of 12 equipment on a boat was designed to work properly using normal discharge levels on FLA batteries.

I get up and start the generator. This starts the re-charge process, but when I wake up everything is fine.

 

[ksanders]

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.

Your problem is not running down your house bank.

The problem is that you have your instruments tied to your start bank which when you turn the key has extremely low resistance circuit until your starter motor starts turning.

This produces an incredible amount of current for a short period of time and that current combined with the normal internal resistance of your batteries produces a low voltage condition which exceeds the specifications of your equipment.

 

[rslifkin]

Your problem is not running down your house bank.

The problem is that you have your instruments tied to your start bank which when you turn the key has extremely low resistance circuit until your starter motor starts turning.

This produces an incredible amount of current for a short period of time and that current combined with the normal internal resistance of your batteries produces a low voltage condition which exceeds the specifications of your equipment.

Agreed. I'm strongly of the opinion that a starting battery starts an engine, runs its gauges, etc. But nothing else. Basically if it doesn't go through the engine key switch, it shouldn't run off a starting battery. Both for preserving starting power and for avoiding starting-related voltage dips causing electronics to reboot.



Thrusters often present the same problem and need their own battery banks. A windlass can as well, but given a big enough battery bank the voltage dip from a windlass may very well be small enough to be a non-issue. My windlass runs off the house bank, for example, and it's never bothered any of my electronics.

 

[ksanders]

Agreed. I'm strongly of the opinion that a starting battery starts an engine, runs its gauges, etc. But nothing else. Basically if it doesn't go through the engine key switch, it shouldn't run off a starting battery. Both for preserving starting power and for avoiding starting-related voltage dips causing electronics to reboot.



Thrusters often present the same problem and need their own battery banks. A windlass can as well, but given a big enough battery bank the voltage dip from a windlass may very well be small enough to be a non-issue. My windlass runs off the house bank, for example, and it's never bothered any of my electronics.

I agree completely! Even in my cabin cruiser days I would install a separate house bank using golf cart batteries and then connect it to the engine battery using an ACR.

My windlass is also on my house bank. I have found that if my bank is discharged it is MUCH better to start my generator when lifting the anchor especially under wind load. If I forget my Navigation electronics will almost always reboot due to the high load producing a low voltage.

 

[eheffa]

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

Thanks Chris,

There is a lot to unpack here.

The housebank voltages do not reflect SOC in any reliable way so I agree that that these voltage / SOC charts are idealized and not reflective of a real housebank that has been in use and may have been degraded or the SOC under load.

The Firefly Carbon Foam (FF) batteries do behave differently from AGM’s.

I made the mistake of setting the charging protocol to have a float charge for the first 5 years of owning our FF’s. . After a few weeks on shore power and first using the house bank we would see a very rapid sag in voltage down to ~ 12.4-12.5 VDC with only 50-ish Ah draw. The voltage would then more slowly decline towards 12.1 VDC as we drew down further. After a re-charge cycle, or two we would start to see a more robust maintenance of voltage with less of an initial sag.

Reviewing the updated FF manual (2020), they suggest avoiding a float charge and using a CC/CV charge protocol with no float. They also describe a conditioning protocol where the batteries are drawn down to 10.5V & recharged as aggressively as possible. I’ve done a variation of this recently (drawn down to ~23% SOC @ 11.3 VDC) and am now seeing much better maintenance of voltage for any given SOC.

So, it looks like my FF bank needed a different charging protocol and at least one conditioning cycle. I think it is functioning much better now with higher voltages for any given SOC. It also appears to be accepting higher currents on the recharge cycle.

Time will tell.

Nevertheless, it’s also good to know that it’s reasonably safe to touch on 12.0 or 11.9V without damaging the instruments.

I do know that the Furuno chartplotter is very sensitive to lower voltages and I wonder whether the NMEA 2000 instruments don’t act up (eg failure of all 3 GPS units to get a fix etc.) when they’ve been subject to lower voltages. So to try to eliminate this risk, I’ve just installed a Victron DC-DC 12-12 18A converter to supply these instruments with steady 13.3 VDC power. It’s too early to say how effective this will be but I’m not provoking a reboot of the MFD anymore when starting the engine. (The addition of the DC-DC unit does not seem to have changed the current draw very much.)

Fingers crossed.

I may be able to hold off on a LiPO4 upgrade for a few more years.


-evan

 

[backinblue]

Another big draw at engine start can be glow plugs/air heating elements for the engine(s) if you have them. They hold down my battery voltage for the first couple minutes of runtime.

 

[SteveK]

If I had thrusters I would run them from the start battery, same as I do with anchor winch as it is unnecessary drain on the house bank, plus can drop voltage for other devices. Now that I think about it must check what powers the dingy davit as that should come from start too.

 

[eheffa]

If I had thrusters I would run them from the start battery, same as I do with anchor winch as it is unnecessary drain on the house bank, plus can drop voltage for other devices. Now that I think about it must check what powers the dingy davit as that should come from start too.

Hi Steve,

I have a separate deep cycle AGMs for the Bow Thruster / Windlass and for the Stern Thruster. Although the draw from these devices is relatively short-lived they are very high current and might be better served by dedicated deep cycle batteries rather than the start battery?

-evan

 

[eheffa]

Your problem is not running down your house bank.

The problem is that you have your instruments tied to your start bank which when you turn the key has extremely low resistance circuit until your starter motor starts turning.

This produces an incredible amount of current for a short period of time and that current combined with the normal internal resistance of your batteries produces a low voltage condition which exceeds the specifications of your equipment.

Hi Kevin,

I have my start battery isolated behind an ACR relay but I can recruit the House Bank to assist in starting if I close the relay. Nevertheless, using the start battery (even with the relay open) seems to cause a sag in the overall house voltage so maybe there is fault there I need to sleuth out. When the relay is open, the respective voltages of the house bank and start battery track independently and separately so the relay seems to be working well by that test.

 

[SteveK]

Hi Steve,

I have a separate deep cycle AGMs for the Bow Thruster / Windlass and for the Stern Thruster. Although the draw from these devices is relatively short-lived they are very high current and might be better served by dedicated deep cycle batteries rather than the start battery?

-evan

Evan. Agree to disagree, each to their own choice. Start batteries are designed for exactly what you described. High current for short time.

 

[backinblue]

Hi Steve,

I have a separate deep cycle AGMs for the Bow Thruster / Windlass and for the Stern Thruster. Although the draw from these devices is relatively short-lived they are very high current and might be better served by dedicated deep cycle batteries rather than the start battery?

-evan

Dedicated batteries are great, but if you don't want to go through that, start bank should be fine. Generally your engine is already started before you use thrusters and windlass. Any draw down should re-charge during running. When returning home, if you use a lot of thrusters docking then shut off your engine, you can plug into shore power to recharge.

 

[eheffa]

Evan. Agree to disagree, each to their own choice. Start batteries are designed for exactly what you described. High current for short time.

Fair enough. I don't disagree I'm only asking the question.

My boat came with the dedicated AGM Deep cycle batteries so never considered changing that configuration. Thrusters and windlass will usually only be used when the engine is running so running off the start battery sounds like a reasonable option too.

-Evan

 

[eheffa]

Dedicated batteries are great, but if you don't want to go through that, start bank should be fine. Generally your engine is already started before you use thrusters and windlass. Any draw down should re-charge during running. When returning home, if you use a lot of thrusters docking then shut off your engine, you can plug into shore power to recharge.

What you say makes sense.

In a bigger boat and 12V systems the longer cable runs will demand pretty hefty gauge wire but otherwise, sure Start battery makes sense.

-evan