question

sweyn avatar image
sweyn asked

Automatic Bilge Pump

I have a Lynx Smart BMS and distributer with LifePo4 and I’m wondering where the best place to connect the bilge pump is.

Before the Smart BMS to guarantee it always works?

Worried that it can then run the battery dead.

lynx distributorbilge
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3 Answers
kevgermany avatar image
kevgermany answered ·

Dead battery or dead battery and sunken boat?

Is there any full time charger?

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sweyn avatar image sweyn commented ·
Valid point.
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Stefanie avatar image
Stefanie answered ·

@Sweyn

You could connect the bilge pump to the AUX terminals. This would require a relay as the AUX terminals max. amperage is 1.1A.

But in case the Lynx Smart BMS opens the contactor (OFF mode) for whatever reason (low cell voltage, user turned the LSB manually off), the AUX output is turned off as well.

I am a boat owner myself and have connected my bilge pumps directly to the battery positive terminal just in case. My bilge pumps protect my ship (not my batteries), which value exceeds that of lithium smart batteries by far.
Sizing of my battery system takes bilge pumps into account and ensure they are are running for at least 48 hours continuously. Enough time to take action.

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seb71 avatar image
seb71 answered ·

Before the Smart BMS to guarantee it always works?

This will not guarantee that the pump will always work.

When the battery is empty, it's empty no matter where you connect the pump.

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sweyn avatar image sweyn commented ·

Factually correct and obvious, but not helpful.

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seb71 avatar image seb71 sweyn commented ·

It was not obvious, because you stated the contrary (which is wrong).

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sweyn avatar image sweyn seb71 commented ·

Everyone, including you, understood what I meant.

Nothing will ever always work, because you could always drop a nuclear device on it.

Pedantry is unhelpful.

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seb71 avatar image seb71 sweyn commented ·

So you think that the Lynx BMS will randomly and needlessly disconnect the battery?

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sweyn avatar image sweyn seb71 commented ·
No, I think the Lynx BMS will disconnect the battery if the voltage gets below a certain point.


The balance of risk is whether killing expensive batteries in the case of a faulty bilge pump is more of a concern than potentially turning off a bilge pump that is keeping the boat afloat to save the batteries.

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seb71 avatar image seb71 sweyn commented ·

No, I think the Lynx BMS will disconnect the battery if the voltage gets below a certain point.

That happens when one or more cells is discharged down to 2.8V. In this situation that battery can not power a pump. The battery will just die. And your boat will still sink.

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sweyn avatar image sweyn seb71 commented ·

Incorrect. The battery will continue to effectively supply a bilge pump well below the voltage where a BMS would disconnect, damaging the battery in the process.

It is perfectly possible that this extra time saves the boat.

Equally, it is perfectly possible that a faulty pump kills the battery.


Balance of risks.

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kevgermany avatar image kevgermany ♦♦ sweyn commented ·
You don't say what other equipment is installed, but with a GX device, battery monitor and an internet connection you can have remote monitoring and alarms.
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seb71 avatar image seb71 kevgermany ♦♦ commented ·

Well, even without those alarms, if the Lynx BMS disconnects the battery, maybe someone notices that the boat is without power.


And I would guess that it's a good idea to occasionally check the water level at the bottom of the boat. The battery could be fine and not disconnected but the pump could be dead. Or something blocking the pump or the pipes.

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seb71 avatar image seb71 sweyn commented ·

As I wrote, Lynx BMS disconnects the battery when a cell gets under 2.8V.


At that point the battery has very little stored energy left.


Just take a look at the discharge curve for LiFePO4: when near empty, there is a rapid drop of voltage, with very little capacity left until the battery is dead (a similar behavior, but in reverse it has when charging and is getting full - once it gets above 3.55V-3.6V you have a rapid raise in voltage and you must stop the charge).

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