Voltage question

[Rosemary]

I have a 2020 R27 OB and this past weekend I used the microwave while I was anchored and I noticed the voltage reading on my Garmin screen was reading 14.5V before turning the microwave on then I watch the voltage drop to 12.0V in less than one minute. After that I started the engine and moved the boat and the voltage reading returned to 14.5V.

What happened and is it an issue to be concerned about?

Thank you

 

[Mike and Sarah]

This is normal as the microwave draws 800 watts.

 

[CruisingElvinRay]

Voltage drop and rebound is normal while operating high wattage devices.


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[knotflying]

Whenever I use a high current consumption device with the inverter I have the engine running.

 

[CruisingElvinRay]

Whenever I use a high current consumption device with the inverter I have the engine running.

Why?


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[Mike and Sarah]

I don’t run the engine when using the microwave. It’s usually on a short time so it’s not a problem. But if I am running the water heater off the inverter that’s a different story. It takes over an hour to heat the tank full. So we only run the water heater off the inverter while underway. We also parallel the house and the engine batteries together. That said you need to be at high cruising speed so the alternator can produce enough power to keep up with what the water heater draw.
I’m not sure if the r23 is the same as my r27 for running the water heater.

 

[Cutwater28GG]

I don’t run the engine when using the microwave. It’s usually on a short time so it’s not a problem. But if I am running the water heater off the inverter that’s a different story. It takes over an hour to heat the tank full. So we only run the water heater off the inverter while underway. We also parallel the house and the engine batteries together. That said you need to be at high cruising speed so the alternator can produce enough power to keep up with what the water heater draw.
I’m not sure if the r23 is the same as my r27 for running the water heater.

Doesnt your boat have a cooling loop from the engine that heats the water tank? it shouldnt need the inverter / water tank heater on if the engine is running.

i get hot water heating as soon as the engine temp is up to 185 and the thermostat opens. doesnt require any electricity.


To the original question, yes thats normal. the microwave is a high power item but only uses power for a short period so in theory your batteries can handle its use on their own.
the 12v batteries in boats will drop voltage when they have a high current draw, the voltage will recover after the draw has been removed but clearly the battery is not as charged as it was before.

the math of the situation is this:
microwave uses 800W of power (P). the formula is P= V x I (V = volts and I = amps of current)

assuming no losses in the cabling and inverter (which isnt true)
therefore if the microwave was used for 1 hour it would consume about 60 amps. (P/V= I)

Battery capacity is measured in Amp Hours = Ah - you typically have about 100Ah on these boats with two 110Ah house batteries in parallel. so you can draw 100amps for 1 hour and then batteries will be below 50% SoC. its less than that in real terms for high current draw usage but we'll ignore that for now)

the point is the microwave is only on for what 3 minutes? so its a Ah is much less than the 60 AMPs
so 60/3 to get .05 of an hour * 60 amps = 3Ah

hopefully this makes sense and I didnt make a mistake above! :shock:

I have simplified the math to get to approximate values

 

[Submariner]

Doesnt your boat have a cooling loop from the engine that heats the water tank? it shouldnt need the inverter / water tank heater on if the engine is running.

i get hot water heating as soon as the engine temp is up to 185 and the thermostat opens. doesnt require any electricity.

Us with the outboard engines don't get hot water from our engine. We have to run the inverter or plug into shore power.

 

[Submariner]

Whenever I use a high current consumption device with the inverter I have the engine running.

Why?

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There's an ACR between the engine and house battery bank. The ACR, when it senses a charging voltage will close putting the engine and house bank in parallel. This allows the engine to charge the house bank. When voltage drops below 12.8, which will happen when you turn on the inverter and hot water heater, the ACR will open, thus isolating the house bank to protect the engine battery.

The hot water heater pulls about 60 amps and it can pull it for awhile (upwards of 90 minutes). With other things running on the boat, this could easily be around 75 amps total being pulled. The best way to get hot water with an outboard is to parallel the engine and house batteries using the emergency parallel switch. This bypasses the ACR entirely and allows the engine alternator to pour those electrons into the house battery and provide for the house loads. At idle, the engine will put in 20amps to help with the 60 amp load. If you're out cruising, up on plane... doing 4500RPM... you'll see the max output of the alternator (I'd guess this to be about 35 amps for an R23, and around 45 or 50amps for the R25 and R27). Having the extra 20-45amps or so from the engine being fed directly to the house bank helps a lot in getting hot water.

Before we did our battery upgrade, we would always leave the dock with a hot water tank full of hot water (using shore power to heat it up). Then as we'd depart, we'd parallel the engine/house, turn on the inverter and power on the hot water tank. We'd cruise like this to our destination which keeps the hot water hot. Once we set anchor and powered off the engine, we'd isolate the house and engine batteries (turn the emergency parallel switch back). The hot water tank would usually last about a day.

It takes about 1600 watts to heat 6 gallons of water from 45 degrees to 170 degrees. The house battery bank with two AGM 110Ah batteries will provide about 1200 watts total.

Also note, because of the voltage drop that's normal when running a heavy load such as the microwave or the hot water heater or the coffee maker, if the batteries were say, only at 70% state of charge, the voltage drop could be such that the inverter beeps at you (low voltage alert) and may shut itself off. Idling the engine with the engine/house in parallel helps with this as well.

This is why running the engine when the hot water heater is on is helpful.

In addition to the 115volt AC loads... I also learned this lesson the hard way with the windlass which is a 12 volt DC load. It pulls a ton of current also. Woke up in the morning after a night on the hook, idled the engine but forgot to parallel the engine/house batteries. As I was pulling the anchor, my wife yells out the window that the chart plotter was rebooting and the VHF radio was turning off/on... low voltage alarms. Batteries were already low on charge from the night at anchor. Then using the windlass caused the voltage to drop below 11.5volts... Paralleling the engine/house batteries allowed the engine to provide the electrical needed and helps.

 

[CruisingElvinRay]

Whenever I use a high current consumption device with the inverter I have the engine running.

Why?

Sent from my iPhone using Tapatalk

There's an ACR between the engine and house battery bank. The ACR, when it senses a charging voltage will close putting the engine and house bank in parallel. This allows the engine to charge the house bank. When voltage drops below 12.8, which will happen when you turn on the inverter and hot water heater, the ACR will open, thus isolating the house bank to protect the engine battery.

The hot water heater pulls about 60 amps and it can pull it for awhile (upwards of 90 minutes). With other things running on the boat, this could easily be around 75 amps total being pulled. The best way to get hot water with an outboard is to parallel the engine and house batteries using the emergency parallel switch. This bypasses the ACR entirely and allows the engine alternator to pour those electrons into the house battery and provide for the house loads. At idle, the engine will put in 20amps to help with the 60 amp load. If you're out cruising, up on plane... doing 4500RPM... you'll see the max output of the alternator (I'd guess this to be about 35 amps for an R23, and around 45 or 50amps for the R25 and R27). Having the extra 20-45amps or so from the engine being fed directly to the house bank helps a lot in getting hot water.

Before we did our battery upgrade, we would always leave the dock with a hot water tank full of hot water (using shore power to heat it up). Then as we'd depart, we'd parallel the engine/house, turn on the inverter and power on the hot water tank. We'd cruise like this to our destination which keeps the hot water hot. Once we set anchor and powered off the engine, we'd isolate the house and engine batteries (turn the emergency parallel switch back). The hot water tank would usually last about a day.

It takes about 1600 watts to heat 6 gallons of water from 45 degrees to 170 degrees. The house battery bank with two AGM 110Ah batteries will provide about 1200 watts total.

Also note, because of the voltage drop that's normal when running a heavy load such as the microwave or the hot water heater or the coffee maker, if the batteries were say, only at 70% state of charge, the voltage drop could be such that the inverter beeps at you (low voltage alert) and may shut itself off. Idling the engine with the engine/house in parallel helps with this as well.

This is why running the engine when the hot water heater is on is helpful.

In addition to the 115volt AC loads... I also learned this lesson the hard way with the windlass which is a 12 volt DC load. It pulls a ton of current also. Woke up in the morning after a night on the hook, idled the engine but forgot to parallel the engine/house batteries. As I was pulling the anchor, my wife yells out the window that the chart plotter was rebooting and the VHF radio was turning off/on... low voltage alarms. Batteries were already low on charge from the night at anchor. Then using the windlass caused the voltage to drop below 11.5volts... Paralleling the engine/house batteries allowed the engine to provide the electrical needed and helps.

Thanks for that detail. You always provide very thoughtful explanations. We usually leave the dock with hot water, and the tanks stays hot (enough) for us until we are on the hook and can run the generator for a while. The microwave is a different story, since it is only used for a few minutes at a time.


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[Cutwater28GG]

Doesnt your boat have a cooling loop from the engine that heats the water tank? it shouldnt need the inverter / water tank heater on if the engine is running.

i get hot water heating as soon as the engine temp is up to 185 and the thermostat opens. doesnt require any electricity.

Us with the outboard engines don't get hot water from our engine. We have to run the inverter or plug into shore power.

Of course silly me.