Bilge pump battery Q?

[er9]

Would it be ok to cut and split my positive wire on my bilge pump into two leads and hook one lead to the house bank and the second positive lead to the start bank? Essentially wiring the pump directly to both banks.

The single ground would go to the ground bus and both branches of the positive wire would be fused on each branch.

If it was connected this way would that cause 1he voltage at the pump to be 24 volts or would it remain 12?

I notmally hook my bilge to the larger house bank but was thinking...what if i was out on a long cruuse and the house bank became very depleted, maybe down to 12.3 volts. What if i had an emergency at yhat moment...it wouldnt give the bilge pumps much energy to use for very long

To avoid this scenariio it seems to me that a long range cruiser should maybe have a moderately sized start bank as a backup, maybe a couple hundred amp hours and to either connect the bilges to that bank as well or be able to switch them to that bank if needed in an emergency.

My tiny little 65 amp hour group 24 is great for starting but wouldnt last long with one or two bikge pumps drawing from it at 20ish amps.

What do you think?

[marshmat]

What you're proposing to do is to permanently wire the starting and house batteries in parallel, via the bilge pump's Y-wire. Any time there is a difference in load on the house vs. starting banks, current will flow from one to the other via this wire. If it's just a little bit of current, both batteries will go dead at the same time. If it's a lot of current, one or both fuses will pop, and you'll be left with no bilge pump.


The point of a separate starting battery is to ensure that you can always start the engine, and therefore generate more electricity, even if the house bank is drawn down to near-empty or if there's a fault somewhere else in the system. If the house bank is too low to run the bilge pumps, the correct response is not to drain the starting battery to run the bilge pumps for a bit longer - it is to start the engine, and let the alternator provide the power for the pumps.


If you are concerned about being without a bilge pump in the event of a main DC bus failure, a common solution is to add a second, totally independent bilge pump, powered from the starting battery via an appropriate circuit breaker. It should either be manual only (no float switch) or else should have both a float switch and a very loud alarm set at a higher water level than the float switch for the main pump. Thus, it will only draw from the starting battery if the main pump has failed or can't keep up, and the alarm immediately lets you know that you must start the engine right away to get the alternator spinning.

[er9]

Originally Posted by marshmat

What you're proposing to do is to permanently wire the starting and house batteries in parallel, via the bilge pump's Y-wire. Any time there is a difference in load on the house vs. starting banks, current will flow from one to the other via this wire. If it's just a little bit of current, both batteries will go dead at the same time. If it's a lot of current, one or both fuses will pop, and you'll be left with no bilge pump.


The point of a separate starting battery is to ensure that you can always start the engine, and therefore generate more electricity, even if the house bank is drawn down to near-empty or if there's a fault somewhere else in the system. If the house bank is too low to run the bilge pumps, the correct response is not to drain the starting battery to run the bilge pumps for a bit longer - it is to start the engine, and let the alternator provide the power for the pumps.


If you are concerned about being without a bilge pump in the event of a main DC bus failure, a common solution is to add a second, totally independent bilge pump, powered from the starting battery via an appropriate circuit breaker. It should either be manual only (no float switch) or else should have both a float switch and a very loud alarm set at a higher water level than the float switch for the main pump. Thus, it will only draw from the starting battery if the main pump has failed or can't keep up, and the alarm immediately lets you know that you must start the engine right away to get the alternator spinning.

Thank you that makes total sense. I wasnt thinking it through completely. I like the idea of the second pump to the other bank with the alarm.

Much thanks

[tkeithlu]

Hmmm. Marshmat gave you a really good answer. I'm thinking one step broader.

Two issues. The first is being able to parallel your starter battery and your house battery in the event that the starter battery is too low to start the engine. You can do that with a rotary 1-2-both switch, or you can simply carry jumper cables on your boat, long enough to reach between the two.

The second issue, rarely addressed, is your batteries being low in the boat and among the first to go with seawater ingress. Suddenly, no bilge pumps, no VHF radio to call for help, sinking boat. My solution to that is a smaller battery behind the instrument panel with a 1-2-both switch that normally (Both) makes it part of the house bank, but which can be isolated to feed the DC buss.

[jmschmidt]

My main bilge pump (2700gal/hr), bilge sump pump (300gal/hr) and my super high-water screaming alarm switch (just below when the floor boards will start to float) are all wired to a "hot battery bus" (airplane talk). They are fail-safe fused separately from these same items which can also be activated on the Blue Sea breaker/switch DC distribution panel which is connected through the master battery switch (Blue Sea) which is off when the boat is unattended (if I don't forget) .

This will ensure that both bilge pumps and the alarm will run until until the water level sensors (solid-state) detect low or no water or the batteries go dead. At the slip the the shore power and battery charger are always on (if I don't forget).

The hot batter bus will also power items like the boat's security system, chart table lighting and the AM/FM stereo which looses its pre-set memory if power is interrupted.

[Boatyarddog]

Think about a backup portable generator
Small easy to store, simple solution.

[captnknopf]

I am surprised no one mentioned using two diodes on the plus lead (with proper fuses) so if one battery goes dead, the other will still work, minus the .7 volt drop across the diode (or .4 if Shotkey)

[er9]

Originally Posted by tkeithlu

Hmmm. Marshmat gave you a really good answer. I'm thinking one step broader.

Two issues. The first is being able to parallel your starter battery and your house battery in the event that the starter battery is too low to start the engine. You can do that with a rotary 1-2-both switch, or you can simply carry jumper cables on your boat, long enough to reach between the two.

The second issue, rarely addressed, is your batteries being low in the boat and among the first to go with seawater ingress. Suddenly, no bilge pumps, no VHF radio to call for help, sinking boat. My solution to that is a smaller battery behind the instrument panel with a 1-2-both switch that normally (Both) makes it part of the house bank, but which can be isolated to feed the DC buss.

my setup is actually in reverse. my larger house bank powers everything through a common bus and it also does starting duty. my start batteries are actually just a reserve bank if the house bank dies. i do use a 1,2,Both switch. in the event my house bank dies i can switch it to #2 bank. My reserve bank is currently two group 24's at 65 AH each so it could run bilge, basic electronics for a bit and still start my engines.

in my situation im fortunate. the builder of my boat put the start battery box (nice heavy fiberglass box) way up behind the engine a foot above the waterline. the top of my house bank is currently about at waterline level. next season when i upgrade to a larger house bank they are going to get placed in a high sided battery box that will be above water level so flooding would never really be an issue unless im sinking.

[er9]

Originally Posted by jmschmidt

My main bilge pump (2700gal/hr), bilge sump pump (300gal/hr) and my super high-water screaming alarm switch (just below when the floor boards will start to float) are all wired to a "hot battery bus" (airplane talk). They are fail-safe fused separately from these same items which can also be activated on the Blue Sea breaker/switch DC distribution panel which is connected through the master battery switch (Blue Sea) which is off when the boat is unattended (if I don't forget) .

This will ensure that both bilge pumps and the alarm will run until until the water level sensors (solid-state) detect low or no water or the batteries go dead. At the slip the the shore power and battery charger are always on (if I don't forget).

The hot batter bus will also power items like the boat's security system, chart table lighting and the AM/FM stereo which looses its pre-set memory if power is interrupted.

what alarm are you using?