How to Use LM3914 Bargraph for 12V/24V/48V Battery SOC and SSR Control for Solar

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#1 21683381 29 Oct 2019 23:11

Edward Chase Edward Chase

Anonymous

Post #1
21683381 29 Oct 2019 23:11

Inspired by Julian Edgar’s EPE article in October’s edition I tried to buy a car battery bargraph display like the one he mentioned, but that worked at 24V and 48V too. I found one that looked similar (but without the USB sockets) and very cleverly detected if the battery was 12, 24 or 48V and adjusted the thresholds to suit.
Unfortunately though, unlike its eBay pictures, it was a completely potted unit so I could not dismantle it to get to the bargraph terminals. What I need it for is to not only show state of charge(SOC)/voltage but also detect when a solar charged set of batteries is nearly fully charged, like SOC >90%, and turn on an immersion heater. I don't want the heater on when battery level is low. Some of the batteries are 24V and some 48V.
I will tap off the highest or nearly highest green segment and use it to turn on a SSR.
What I have done instead is to buy a LM3914 PCB as per http://www.onstatetech.com/Tech-specs/Bargraph-meter%20BM-7%20basic%20DC%20schematic.pdf and will populate it myself, with some fitted with resistors for 24V and some for 48V.

Does anyone have any tips of getting it to accurately detect the voltage? I.e. resistor and temperature drift may mean that the detection of SOC may be variable.
How accurate is a LM3914 in the 20 -40C range this will operate in?
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#2 21683382 31 Oct 2019 12:58

Maurizio Di Paolo Emilio Maurizio Di Paolo E...

Anonymous

Post #2
21683382 31 Oct 2019 12:58

You should need of battery monitoring such as BQ79606A-Q1. I think that there are dev kits for that.
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#3 21683383 31 Oct 2019 21:17

Elizabeth Simon Elizabeth Simon

Anonymous

Post #3
21683383 31 Oct 2019 21:17

Unfortunately I haven't got the time to look at the LM3914 data sheet in detail but with other applications where I've had to use resistor dividers to measure voltage over temperature, I've found that I needed higher precision resistors. In particular, you need to look at the temperature coefficient.Most 1% resistors are 100ppm/degC. You can calculate the effect of temperature and tolerance using the following formulaR = Rnom * (1 +/- tol) * (1 +/- (tempco * deltaT))where deltaT is the difference between the min or max temperature and the nominal temperature (25C).You'll want to use a spreadsheet or MathCad or some such for these calculations.
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#4 21683384 02 Nov 2019 02:13

Julian Edgar Julian Edgar

Anonymous

Post #4
21683384 02 Nov 2019 02:13

I am glad I inspired something! In your situation I'd use multiple prebuilt eBay modules. eg multiple LED voltmeters for the batteries, and a voltage switch (or multiple switches, as required) to operate the heater. Some switches also include a voltmeter eg https://www.ebay.co.uk/itm/DC-12V-Voltage-Detection-Charging-Discharge-Relay-Switch-Control-Board-Module/253027302034?hash=item3ae99a3a92:g:LkUAAOSwwvZZW1qP
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#5 21683385 02 Nov 2019 22:35

Edward Chase Edward Chase

Anonymous

Post #5
21683385 02 Nov 2019 22:35

Julian, was not aware of that type of module. That would indeed
be better for me. Although it can’t be powered from more than 16V is does
measure up to 99V. But a simple 7812 regulator circuit should manage the power.
Can set the hysteresis by selecting upper and lower thresholds. The timer
function may possibly also be useful to me to say turn on the load for
enough time to run say a washing machine cycle, but so long that a lot of
battery power is used.

https://www.ebay.co.uk/itm/6-80V-Voltage-Dete...=item5b6111810a:g:1QgAAOSwPbddnA2w seems
an alternative. Documentation not so good and no timer but can be powered from
up to 80VDC and has a nice looking case.

I guess if I was determined I could use a resistor divider on
the measurement input with a pair of resistors of different temperature
coefficients so the set point varied in according with a lead acid battery’s
shift of SOC point. But would have to first measure what drift in setpoint is
caused by the module’s own circuit.

This isn’t for SOC but it gives an idea:
Can't get the graph to post without error messages but it came from https://www.solar-electric.com/learning-center/deep-cycle-battery-faq.html/
#6 21683386 14 Nov 2019 13:17

Joe Semos Joe Semos

Anonymous

Post #6
21683386 14 Nov 2019 13:17

Can you show the Bar graph, I didn't understand the scenario by your theory.
#7 21683387 12 Dec 2019 10:37

Anthony Kristina Anthony Kristina

Anonymous

Post #7
21683387 12 Dec 2019 10:37

Julian, was not aware of that type of module. That would indeed be better for me. Although it can’t be powered from more than 16V is does measure up to 99V. But a simple 7812 regulator circuit should manage the power. Can set the hysteresis by selecting upper and lower thresholds. The timer function may possibly also be useful to me to say turn on the load for enough time to run say a washing machine cycle, but so long that a lot of battery power is used.
https://www.ebay.co.uk/itm/6-80V-Voltage-Dete...=item5b6111810a:g:1QgAAOSwPbddnA2w seems an alternative. Documentation not so good and no timer but can be powered from up to 80VDC and has a nice looking case.
I guess if I was determined I could use a resistor divider on the measurement input with a pair of resistors of different temperature coefficients so the set point varied in according with a lead acid battery’s shift of SOC point. But would have to first measure what drift in setpoint is caused by the module’s own circuit.
This isn’t for SOC but it gives an idea:
Can't get the graph to post without error messages but it came from https://www.solar-electric.com/learning-center/deep-cycle-battery-faq.html/
Love it thank for the valuable information...
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Topic summary

The discussion centers on using the LM3914 bargraph IC to monitor state of charge (SOC) and voltage levels of 12V, 24V, and 48V solar-charged batteries, with the goal of controlling a solid-state relay (SSR) to activate an immersion heater only when the battery SOC exceeds approximately 90%. The original poster sought a multi-voltage bargraph display capable of auto-adjusting thresholds for different battery voltages but found commercial units to be potted and inaccessible for modification. Instead, a DIY approach using an LM3914 PCB was pursued. Key technical considerations include precise voltage measurement using resistor dividers with low temperature coefficient resistors to maintain accuracy over temperature variations, and the use of voltage detection relay modules as alternatives for switching loads based on battery voltage thresholds. Suggested solutions include employing multiple LED voltmeters combined with voltage switch modules or dedicated voltage detection charging/discharging relay boards capable of operating up to 80-99V with adjustable hysteresis and timer functions. Examples of such modules were referenced from eBay listings, including DC voltage detection relay switch control boards and voltage monitoring relay switch controllers. The importance of temperature compensation in resistor selection and the potential use of resistor pairs with differing temperature coefficients to track lead-acid battery SOC shifts were also discussed. Additionally, a recommendation was made to consider integrated battery monitoring ICs like the BQ79606A-Q1 with available development kits for more advanced battery management.
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How to Use LM3914 Bargraph for 12V/24V/48V Battery SOC and SSR Control for Solar

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