Overvoltage protection 48V system

[Romszy]

Hi all,

I am new to the forum so please excuse the fact that this may have already been discussed. I'm struggling to find a simple solution to my problem but i know it exists!

I am charging some very sensitive batteries using a generator attached to a diesel engine. The issue is that the voltage of the generator varies with engine RPMs so there is a possibity that someone engages the generator with high engine rpms which in turn damages the batteries.

I already have a battery disconnect relay which works on a low current DC signal. So here is the question:

I am looking for an instruments which monitors the voltage ( nominal voltage is 48V ) and if that voltage goes over a certain threshold ( ideally this would be programmable), it sends a low current DC signal to the relay. Should be super simple but can't find anything obvious.

Any help would be grately appreciated.

Rom

 

[Nigel Goodwin]

For a start, what kind of batteries are they? - and why are they 'super sensitive'?

 

[Romszy]

For a start, what kind of batteries are they? - and why are they 'super sensitive'?

They are lithium batteries and have their own internal protection but in the interest of weight saving they are protected by diodes which need to be replaced in the event of an overvoltage. this is fairly time-consuming so was hoping to have a system able to interupt the charge before this stage. The voltage range they are able to acccept make them sensitive.

 

[Nigel Goodwin]

You could use a differential amplifier to compare the voltage from the generator with a preset value, which you could adjust with a potentiometer.

There's an example here:

https://www.electroschematics.com/power-supply-voltage-monitoring/

 

[Romszy]

You could use a differential amplifier to compare the voltage from the generator with a preset value, which you could adjust with a potentiometer.

There's an example here:

https://www.electroschematics.com/power-supply-voltage-monitoring/

Thank's i'll look into this. I'll need to find one able to go to 48V. Which then sends a 24V signal to tell the relay to shutoff ( with a potentiometer). I'll see if i can find a detailed example.

 

[danadak]

A possibility. You would use a voltage divider at input to meet its
range.

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Regards, Dana.

 

[Tony Stewart]

Lithium batteries need a proper CC, CV, 5%CC cutoff regulator and BMS for longevity.
You'll want this to avoid explosive situations.

Show datasheet for batteries.

 

[Nigel Goodwin]

Thank's i'll look into this. I'll need to find one able to go to 48V. Which then sends a 24V signal to tell the relay to shutoff ( with a potentiometer). I'll see if i can find a detailed example.

If your relay is 24V, then presumably you've got 24V power? - you could use that 24V to power the monitoring circuit. Or, simply use a voltage regulator to drop the 48V down to where you need.

 

[Tony Stewart]

If they are 12S Lithium Ion 48V or 4.0V/cell ought to be the max float charge voltage as the battery degrades faster above this and more rapidly above 4.2 /cell. Same for undervoltage <36V or < 3V/cell

This is easily accomplished with x% hysteresis to avoid rapid oscillations once you define what stable supply voltages are avail. and max battery charge current, as well as max surge current with contact closure.

You can get an optimal design with specs.

 

[rjenkinsgb]

There is an industrial type here that looks suitable - the "36V" version would cover either low or high limits for the 48V battery, or the 48V type just the high limit.

DC Voltage Monitoring Relay, Under/Over Voltage, 12V/24V/48V DC

DC voltage monitoring relay with 2 C/O contacts (2 Form C or DPDT) provide protection for the equipment from undervoltage and overvoltage in DC circuit 12V, 24V, 36V or 48V, adjustable trip delay time, LED indicator for working state, DIN rail mounting, compact design, good quality and low price.

www.ato.com

The charge limit one should run from the generator side of the charge disconnect relay.

If you another one as a low voltage load disconnect, it should run from the battery side of the load disconnect relay.

 

[Tony Stewart]

Beware of burning relay or solenoid contacts when Generator voltage is mismatched with battery.