Replacing the power on relais with a battery shut off

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Adrian Harris
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Re: Replacing the power on relais with a battery shut off

Post by Adrian Harris » Sat Jan 02, 2021 10:36 pm

It will depend on the make and type of your batteries.

Once you've been running it for a while you'll get to know when it starts to get sluggish, which is your cue to recharge or load it up.

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Gerhard Michel
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Re: Replacing the power on relais with a battery shut off

Post by Gerhard Michel » Sat Jan 02, 2021 10:57 pm

Hi Roy,

lead acid batteries unfortunately have no fix correlation between voltage and remaining capacity, because the voltage is also dependent from load current. Maybe a battery shows 24 volts and ist nearly empty, when measured without current, and another battery has 20 volts and half its capacity remaining, while it is measured under heavy load.

A reliable method to evaluate the battery level of a lead acid battery is to measure the consumed capacity since last fully charging with telemetry. If your battery hat 40 Ah and you consumed 20 Ah, you may be absolutely sure that the tank will reach the base. Why leaving 20 Ah? The nominal capacity of a lead acid battery is measured at 10 hours or 20 hours of discharge. In a tank it will be discharged within 2 to 4 hours. At this quick discharge no lead acid battery reaches its nominal capacity. But the heavier reason for leaving half the capacity is the fact, that the depth of discharge (DoD) should not be more than 50 % when using a lead acid battery for reaching its optimal life time. This value may be exceeded, depending on shortening of life time. Therefore there exists a certain margin for additional loads at an expense of lifetime lacks.

A LiFe battery can endure nearly 100 % DoD without lifetime lacks, when the monitoring of each single cell (!) is good enough. But here is NO MARGIN for additional loads when using the manufacturer's limits. If you want a margin for additional loads, you have to use a warning voltage value some above the absolute limit. LiFes have a fixed correlation between voltage and charging state, so you may avoid measuring of the consumed capacity.

A LiFe battery with BMS may 'final stop' without a visible reason, e.g. when the load current exceeds the BMS limit for an extremely short time.This results in a widely safe use of the battery, but also a 'dead' model in the field. It can only be reactivated by a short charging.
kind regards
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Roy Beukeveld
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Re: Replacing the power on relais with a battery shut off

Post by Roy Beukeveld » Sun Jan 03, 2021 6:45 pm

Hi Gerhard,

Thank you for explaining one and other, I really appreciate it.
I was thinking that probably the battery voltage without current could be an indicator but there I am wrong.
Since I already purchased my batteries (lead acid, 12V, 22ah, rec22, Yuasa) I'll stick with those for now, but the benefits you explain are very clear.
Monitoring the consumed capacity is definitely the way to go. I plan on using the Spektrum SPMA9605 for that.

Thanks for the help :D
Kind regards,

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