areddon
Newbie level 4
I just got some great help on this forum to resolve a logic gate issue, so I thought I would try my luck with what I hope is my last issue.
Below may be too much context and you may ant to skip to my "question" at the end.
For background, the context is a battery management system for a 200 Ah LiFePO4 marine battery bank. The system has a commercial battery management board, an Arduino to refine a couple of functions that the board does not allow, and 7 relays. Control signals are all 5v DC.
To finalize my (now working) prototype I want to trying adding a very simple RC delay in one of the relay control circuits.
The relay in question is a 12V, 120 Amp DC relay (HDC60D160). It is the main disconnect to separate both the charging bus and the load bus from a LiFePo4 battery bank. It is controlled by the battery management board.
Most of the "normal" scenarios for disconnecting the buses from the lithium bank are programmed with a short delay after the triggering condition and before disconnect. This delay allows relays to disable the charging sources (alternator field current, solar charge controller, etc.) a few seconds before the battery disconnect. This prevents voltage spiking from disconnecting the charging sources (most worrisome is 100A alternator) from the battery while under load.
Unlike the delay in the "normal" disconnect scenarios, the emergency disconnect scenarios shut down the charging sources at the same time as the main disconnect is opened. This is probably safe enough, since earlier layers of protection should have shut down charging sources long before emergency conditions are reached and because there is a 12v lead-acid battery in the loop to absorb any unintended/unforeseen transients resulting from main bank disconnects.
That said, I would prefer to introduce a delay into the main disconnect control circuit if it can be done simply.
The disconnect relay is normally open and is held closed during normal operation by a 12 volt signal from the drain of a p-channel MOSFET (STP10P6F6).
The 12v control signal (current) goes to zero in a number of ways, including:
a) There are thermal shut offs on the individual battery cells, and wired in series into the control line,
b) there is a manual emergency shut off button in the control line, and
c) the source of the 12v control signal is a p-channel MOSFET whose drain goes to zero under the control of the battery monitoring module under emergency conditions.
In each case the 12v control signal to the relay is interrupted and the main disconnect relay opens.
My question: Is there a simple resistor-capacitor type arrangement I could put in the wire that carries the +12v control signal in order to cause a few seconds delay in the opening of the relay? The "must turn off" voltage of the relay is 4.2 so if there is a resistor-capacitor arrangement that would take 5-10 seconds to decay from 12 to 4.2, that would be perfect. If a general schematic were provided I could probably figure out the values of R and C that are need from the currents in the data sheet (although I would be happy to hear that to). What I really need is to know if this can work for DC and the requisite arrangement of the elements.
From my hazy 35 year-old memories of a circuits course I once took, I think that this must be an unbelievably simple problem (a resistor in series and a capacitor branching off to ground?) but I cannot figure it out from internet sources.
I attach a rough sketch of what I have now.
P.S. I don't want to use IC timers or a time-delay relay, so I am just looking for a passive, simple, physically robust solution if it is possible
Thanks in advance for your help.
Below may be too much context and you may ant to skip to my "question" at the end.
For background, the context is a battery management system for a 200 Ah LiFePO4 marine battery bank. The system has a commercial battery management board, an Arduino to refine a couple of functions that the board does not allow, and 7 relays. Control signals are all 5v DC.
To finalize my (now working) prototype I want to trying adding a very simple RC delay in one of the relay control circuits.
The relay in question is a 12V, 120 Amp DC relay (HDC60D160). It is the main disconnect to separate both the charging bus and the load bus from a LiFePo4 battery bank. It is controlled by the battery management board.
Most of the "normal" scenarios for disconnecting the buses from the lithium bank are programmed with a short delay after the triggering condition and before disconnect. This delay allows relays to disable the charging sources (alternator field current, solar charge controller, etc.) a few seconds before the battery disconnect. This prevents voltage spiking from disconnecting the charging sources (most worrisome is 100A alternator) from the battery while under load.
Unlike the delay in the "normal" disconnect scenarios, the emergency disconnect scenarios shut down the charging sources at the same time as the main disconnect is opened. This is probably safe enough, since earlier layers of protection should have shut down charging sources long before emergency conditions are reached and because there is a 12v lead-acid battery in the loop to absorb any unintended/unforeseen transients resulting from main bank disconnects.
That said, I would prefer to introduce a delay into the main disconnect control circuit if it can be done simply.
The disconnect relay is normally open and is held closed during normal operation by a 12 volt signal from the drain of a p-channel MOSFET (STP10P6F6).
The 12v control signal (current) goes to zero in a number of ways, including:
a) There are thermal shut offs on the individual battery cells, and wired in series into the control line,
b) there is a manual emergency shut off button in the control line, and
c) the source of the 12v control signal is a p-channel MOSFET whose drain goes to zero under the control of the battery monitoring module under emergency conditions.
In each case the 12v control signal to the relay is interrupted and the main disconnect relay opens.
My question: Is there a simple resistor-capacitor type arrangement I could put in the wire that carries the +12v control signal in order to cause a few seconds delay in the opening of the relay? The "must turn off" voltage of the relay is 4.2 so if there is a resistor-capacitor arrangement that would take 5-10 seconds to decay from 12 to 4.2, that would be perfect. If a general schematic were provided I could probably figure out the values of R and C that are need from the currents in the data sheet (although I would be happy to hear that to). What I really need is to know if this can work for DC and the requisite arrangement of the elements.
From my hazy 35 year-old memories of a circuits course I once took, I think that this must be an unbelievably simple problem (a resistor in series and a capacitor branching off to ground?) but I cannot figure it out from internet sources.
I attach a rough sketch of what I have now.
P.S. I don't want to use IC timers or a time-delay relay, so I am just looking for a passive, simple, physically robust solution if it is possible
Thanks in advance for your help.
