Acceptable minimum voltage during a high inverter load.

[JDCAVE]

What would be he minimum acceptable voltage on a 12 volt house bank on a high inverter load from a 1000 watt microwave event of 30 seconds to 4 minutes? My aging bank, otherwise showing acceptable accounting of volts vs SOC is sagging to 11.92 volts during an high load inverter event.

Jim

 

[rslifkin]

As long as the inverter and any other devices running are fine with voltage in the high 11s, I wouldn't be particularly worried about that. I've seen mine down to 11.8 under heavy load before.

 

[ksanders]

11.92 volts is just fine.

The best way to tell when it's time to replace your house bank is look at either the high load voltage which you are doing, and or look at the voltage when the bank is depleted.

For example Crown rates my FLA bank at 11.74 volts at 50% State Of Charge with a 20 hour load on the bank at that time.

When i start seeing a significantly lower voltage than that it will be time to start thinking of replacement.

 

[w8n4sun]

Ksanders, does that mean after 20hrs being on anchor, you use the microwave and see voltage drop to 11.74?

 

[OldDan1943]

When the microwave shuts down after 30sec use, does the battery voltage bounce back over 12.5 volts?

Gotta have healthy batteries. Tell me the number of house batteries and rated at how many Amp hours and age.
Run your main engine to spin you alternator to put a charge into the batteries at the same time. If you are tied to the dock and shore power, what size is your charger?
We sometimes expect more out of the electrical system that the design
If you had 6 healthy 8D house batteries and a 5000 watt inverter and perhaps a 300 amp alternator on your main engine, I doubt if you would experience a voltage drop.
Advice for the real world, when trying to heat your coffee, use the microwave sparingly.
With a usual set up, 2 or 3 4D house batteries, 2000 watt inverter, 150 amp alternator in your main engine.... You wont be able to make a baked potato.
Start the generator, use the AC to cook to your heart's content, almost.

 

[JDCAVE]

Thanks all. Yes, the voltage bounces back. I did some tests before we left the dock this year and satisfied myself that I was fine for another year.

I have 10 T105’s, 1125 ah. I rarely go below 90% SOC, with solar, and use the Northern Lights to give the batteries a bang 1st thing in the am. Solar brings it up to 100% SOC with a decent threshold charge. Magnum 2812 inverter/charger.

So the question relates to morning coffee, with minimal solar at that time of day. My wife likes her milk for the coffee marked. All other voltages relate to a 5-10 amp load with fridge and freezer. With these off, the meter rises to 12.65 quickly.

I’m satisfied for now. Probably replacing them next year. Probably going down to 6 or 8 GC’s, unless I commit to LFP.

Loosing cell coverage. Talk later.

Jim

 

[OldDan1943]

JDCAVE,
You could time the coffee making for when you have the generator running each morning to charge the batteries.
How old are your batteries now?
I assume the start batteries are not part of the house system. The generator battery is independent to the house batteries too.
Even if you drain your house batteries over night, you can still start your generator to recharge the batteries and make coffee at the same time.
If you are in the habit of starting your main engine and you have it piped to heat the water in the tank, you can shut off the electric to the HW tank too.

As I recall, I can go almost 2 day on the 3X4D batteries. (3X200 AGM) Of course all I have running it the 12 vt fridge and lights and a couple of small inverter to run the TVs.
I do have 2X130 solar panels to help out.
My inverter is only 1800watt inverter but something about a +10% buffer.
It is fun/scary, when I put the microwave (1200 watt) on the inverter, watching the house battery voltage take a nose dive. The voltage always jumps back up to an acceptable range if I am careful.

I doubt, in my life time, will we see our boats living totally off the grid for extended periods of time. (no engine, no generator)
Even the "test" total electric boats, have a forest of solar panels, a mountain of batteries and a generator.

 

[magna 6882]

Basically all your doing is a load test on the battery. Depending on the amp size of the bank and the temp you could see voltage at 9 to 10 volts.What really determines the battery health is bounce back. You would like to see 75 percent or better after 2 or 3 minutes of sitting at rest. Look up load testing and it will help you understand what you should be seeing. I have seen battery banks 10 years old and still passing a load test with specific gravity and voltage pass with correct temps corrected. The trick is to keep them charged. With boats and rvs the sit off season which just kills a battery and invites sulfation .

 

[JDCAVE]

Yes, the voltage comes back up quickly. The bank is on its 8th season. Drinks more water than before. Depending on our plans I will likely replace it before next season.

Separate start batteries, generator start, and thruster bank.

Jim

 

[OldDan1943]

Yes, the voltage comes back up quickly. The bank is on its 8th season. Drinks more water than before. Depending on our plans I will likely replace it before next season.

Separate start batteries, generator start, and thruster bank.

Jim

4D or 8D batteries? Total amps.
Have you considered AGM batteries? (no water usage)

And now you can expect others' to endorse their types of batteries. SMILE

If you get too exotic you may have to change out the charger too (more money)

 

[JDCAVE]

10 T-105’s, see above. I don’t mind watering batteries. It only takes a few minutes every couple of weeks when cruising or couple of months while at shore. I will probably go down to 8 or even 6 batteries. We just aren’t using the capacity we have right now.

The advantageous features of LFP are compelling, however I remain concerned about risks of failure due to over discharge and over charge and also failure of the alternator. I’m not sure the increased costs are worth the increased risks. Perhaps I’ll change my mind next year.

Jim

 

[OldDan1943]

JDCave, it is better to have too many amps than not enough, especially if you are using your microwave for more than 30seconds.

 

[dhays]

JD, I have the same inverter as you. I have it set to stop inverting if the voltage drops too low. However, if in the morning after keeping the fridge, freezer, lights etc going over night, the Keurig could make the voltage drop enough temporarily to trip the low voltage cutoff.

I haven't found a good answer to the problem other than turn off the low voltage cutoff, which I would rather not do.

 

[OldDan1943]

LOL
I have a generator and I am not afraid to use it!!!
Start the generator, start charging your batteries, make breakfast, heat the shower water, run the A/C or heat..... Get everything ready for the day's movement/trip then, shut down the generator if you wish, the generator will use than a gph.
Fuel is the least expensive item of owning a boat.

 

[ksanders]

Ksanders, does that mean after 20hrs being on anchor, you use the microwave and see voltage drop to 11.74?

No that means that after all night at anchor I wake up and the voltage is around 11.74 or thereabouts and the SOC meter is showing around 50% remaining charge.

My boat is always drawing 35-40 amps. Lots of things running all the time.

I have seen around this same voltage and SOC overnight with Two different house banks (interstate FLA and Crown FLA) and Two different SOC meters (Xantrex and victron)

 

[Simi 60]

No that means that after all night at anchor I wake up and the voltage is around 11.74 or thereabouts and the SOC meter is showing around 50% remaining charge.

My boat is always drawing 35-40 amps. Lots of things running all the time.

I have seen around this same voltage and SOC overnight with Two different house banks (interstate FLA and Crown FLA) and Two different SOC meters (Xantrex and victron)

Have you checked individual batteries?

We have a similar thing going on with our 5 year old 880ah 24v system. (8 X 220ah AGM)
Boat pulls anywhere between 5 and 20 Amps at night
Batts have always been at around 80% in the morning (20% used)
Shows around 180ah gone
Yet now, the batts are showing around 23v, used to always be over 24v

If I check individual batts, some are at 12.5v, some are at 11.5v.

Start Genset and almost instantly back over 24v
No real temp difference between batteries as they charge up.
All get to full charge as we have 2500watts of solar to finish them

I thought they were dead - maybe not?

 

[JDCAVE]

Simi: in your particular situation, I think you may have one or more batteries that have a bad cell. You may want to test this by shutting down the system and disconnecting the batteries. They should be within a 0.1 volt of each other. Any less than 12 volts are likely “in a bad way”. The problem is the other batteries will be transferring amps to the dying batteries, rather than to the system. Just my thoughts.

Jim

 

[Simi 60]

Simi: in your particular situation, I think you may have one or more batteries that have a bad cell. You may want to test this by shutting down the system and disconnecting the batteries. They should be within a 0.1 volt of each other. Any less than 12 volts are likely “in a bad way”. The problem is the other batteries will be transferring amps to the dying batteries, rather than to the system. Just my thoughts.

Jim

Of that I am sure but what is the solution?

If it's 1 or 2 batts in a bad way and I was to take two batteries out I have instantly reduced the capacity of the system by 25%

If I leave them in, sure, they ain't as good as they should be and they are dragging others down but surely they are adding "something"? (My logic may be flawed)

I am currently nutting out an LFP upgrade but that'll be 2 months out
In the meantime it's a generous head start of Genset in the morning and if a cloudy day, a top up at dusk to keep it going until we get back to where the new batts will be delivered.

 

[rslifkin]

If one or 2 batteries are failing, you're probably better off taking them out of the bank. Yes, the bank will be smaller, but it may perform better without the bad batteries in the mix.

 

[ksanders]

Have you checked individual batteries?

We have a similar thing going on with our 5 year old 880ah 24v system. (8 X 220ah AGM)
Boat pulls anywhere between 5 and 20 Amps at night
Batts have always been at around 80% in the morning (20% used)
Shows around 180ah gone
Yet now, the batts are showing around 23v, used to always be over 24v

If I check individual batts, some are at 12.5v, some are at 11.5v.

Start Genset and almost instantly back over 24v
No real temp difference between batteries as they charge up.
All get to full charge as we have 2500watts of solar to finish them

I thought they were dead - maybe not?

The manufacturers specification for my current FLA bank indicates a 50% discharge depth is equal to 1.94 volts per cell.

No I have not measured each 6 volt cell because there is no need to... My bank is meeting the manufacturers specification.

It is also note worthy that I have had Two banks with the same characteristics from two different manufacturers using two different brands of measuring equipment.

My FLA bank is doing what FLA banks do when working properly.

 

[JDCAVE]

If one or 2 batteries are failing, you're probably better off taking them out of the bank. Yes, the bank will be smaller, but it may perform better without the bad batteries in the mix.

Agreed. The bad batteries are parasitizing the good batteries. They do not contribute the capacity of the bank. If in fact they are bad. A one volt difference between batteries is sizeable.

Jim

 

[Simi 60]

If one or 2 batteries are failing, you're probably better off taking them out of the bank. Yes, the bank will be smaller, but it may perform better without the bad batteries in the mix.

Thanks
The words probably and may makes it sound a bit swings and roundabouts.
Quite a bit of effort to achieve and in a single micro human* sized space

New batteries will get repositioned in the old cold room beside the inverter/charger
Plenty of space for a crowd.

* Are we allowed to say midget any more?