Battery float with current limiter

[paulca]

Hi,

I want to use an independent SLA battery to power my dashcam in "Parking mode".  This is as opposed to running it off a battery live fuse and using a low voltage cut out to prevent draining the car battery.

So far, from the bit of electronics experience I have, I know that simply placing the battery in circuit with the dashcam the battery will charge while the car is running and the circuit is being supplied with 14.4V, when the car is off the SLA will supply 12V to the dash cam.

The trouble is that in my use case it is highly likely the dashcam will discharge the SLA over the weekend or if the car is laid up for more than a day or two.  When the car comes back on the discharged SLA will have low resistance and V=IR will try and draw too many amps and blow my utility socket 10A fuse, not to mention potentially damage the battery or the connecting wires.

So I believe I need a current limiter as a minimum.  I'm thinking of something like a 4Ah battery, so would expect a maximum charge current of half of that, 2A.

I looked at an LM338 / LM817(?) but I wasn't confident in understanding the typical application circuit in the data sheet.  That would be my first question:  Is the LM388 suitable for a 2A limiter and how would one set it up.  My input voltage will be more-or-less fixed, either 0V or 14.4V.

Then it occured to me if the battery draws >2A the limiter will liimt to 2A and the dash cam (drawing 0.2A) will potentially cut out until the SLA charges up a bit.  This is unacceptable.

So my second question.  Is there a way to allow current to flow from either the utility socket or the battery to the dash cam, but to limit the current from the utility socket to the battery only.  Some form of op amp buffer or something?

[paulca]

My other option is an off the shelf charger, but hunting for a 12VDC Input SLA Charger either ends up expensive, from an international seller or an RC multicharger that would need poked at to put it into lead acid mode and start the charge ( I already have one of these at home)

[David Hess]

I looked at an LM338 / LM817(?) but I wasn't confident in understanding the typical application circuit in the data sheet.  That would be my first question:  Is the LM388 suitable for a 2A limiter and how would one set it up.  My input voltage will be more-or-less fixed, either 0V or 14.4V.

The dropout voltage of the 317/350/338 and most other integrated regulators will be way too high for this application plus their reference voltage is in series with their dropout voltage.

There might be some lithium battery charge controllers suitable for this application.

So my second question.  Is there a way to allow current to flow from either the utility socket or the battery to the dash cam, but to limit the current from the utility socket to the battery only.  Some form of op amp buffer or something?

If the dash camera is directly connected to the SLA battery, then placing the current sense in series with the SLA battery instead of in series with the input will allow the current regulator to ignore the current drawn by the dash camera.  Then the regulator will power the dash camera but apply a controlled charge to the battery.  However if the current drawn by the dash camera is small compared to the maximum charge current which is likely, then it can be ignored.

Note that the charging voltage requirements for a SLA battery are different than for a flooded cell starter battery.  The maximum charging voltage is lower which is actually a good thing because it allows some leeway in the dropout voltage of the regulator.

A low voltage disconnect should still be used with the SLA battery because they are also damaged by excessive discharge.

The simplest possible design and probably what I would go with is a simple low dropout external PNP voltage regulator using beta current limiting which requires no sense resistor like the Linear Technology LT1123.  The circuit would need to be modified to handle the input voltage being disconnected, maybe by adding a germanium or schottky power diode in series with the input.  That still leaves including a low voltage disconnect though.

[paulca]

Thanks David.

I might put this down to a project for later on in my electronic development.

The part that stings is that "BlackVue" who make my dashcam now offer a product that does exactly what my ideas was...  a battery backup for parking mode that avoids using the car battery.  Why does it sting?  It's £190 for a 12 hour run time

[Mjolinor]

If you flatten a lead acid battery then it cannot recover without a lot of work. Anyone that says they are OK is wrong. If you let it get below 10 volts it will not recover. Use a lithium battery and associated circuitry.

[David Hess]

If you flatten a lead acid battery then it cannot recover without a lot of work. Anyone that says they are OK is wrong. If you let it get below 10 volts it will not recover. Use a lithium battery and associated circuitry.

Lithium batteries are not any better in this respect and the lithium polymer ones are worse.  NiCd and NiMH cells may be discharged to zero as long as they are not allowed to reverse.

In practice it is just better to include a low voltage shutoff circuit to protect the battery.

[Zero999]

I agree: use a low voltage cut-off circuit.

A resistor can be used to limit the current drawn, when the battery voltage is low. To minimise unwanted voltage drop, use a resistor with a negative temperature coeffient, so the resistance drops, as it heats up. These can be purchased as inrush current limiters.

[macboy]

If you flatten a lead acid battery then it cannot recover without a lot of work. Anyone that says they are OK is wrong. If you let it get below 10 volts it will not recover. Use a lithium battery and associated circuitry.

Lithium batteries are not any better in this respect and the lithium polymer ones are worse.  NiCd and NiMH cells may be discharged to zero as long as they are not allowed to reverse.

In practice it is just better to include a low voltage shutoff circuit to protect the battery.

You are right about Lithium. Just as bad, but even worse because they can become downright dangerous if charged after being abused.

Right about NiCd and NiMH too. The problem is that these are invariably made into packs of between 6 and 20 cells. The manufacturers almost never go through the exercise of matching them for capacity, so there will usually be "runt" cells in the pack. Perfectly good cells, just maybe 5% lower capacity than the average. The problem is that the pack is usually allowed to continue to discharge after those one or two cells drop to zero, and then they begin to reverse. In no time, the whole pack is buggered because those runt cells die.

Fast charge on SLA battery is usually 1/10 capacity unless specified otherwise. Don't assume 1/2 capacity is OK.

[Zero999]

How abourt using a high powered 6V halogen lamp connected in series? It won't drop much voltage, when the camera is running, but will when the battery voltage is low.

https://www.amazon.co.uk/20W-Halogen-Light-Bulb-Capsule/dp/B005RNHWBA/ref=sr_1_1/259-9458581-1355061?s=lighting&ie=UTF8&qid=1509487866&sr=1-1&refinements=p_n_feature_keywords_three_browse-bin%3A2029905031

[paulca]

There might be some lithium battery charge controllers suitable for this application.

Spending a little more time on this option seems to be the cleanest.  Also, I could use a LiPo which I had excluded due to SLAs being more forgiving and possibly chargeable via the car 14.4V DC directly.

I have many LiPos and in particularly a pair of 5AH 3S blocks that would give 10Amp Hour in parallel as a 3s2p set.

However it's a struggle to find an IC capable of:

Charge current >= 1A ideally (given the cells above) 10A
Charge voltage = 12.6V
Input voltage: 14.4V

As they typically have significant voltage drops a lot are quoting things like 30V input voltage, but support 4 series lipo.   That would be fine if they contained a boost converter.... or I could get both a boost converter to get 30V for the LiPo controller.

The switching between charge and discharge is not critical and I could use a relay for isolation.  The dash cam is super cap powered, so it will stay running for 3-4 seconds so I can switch over with a DPDT relay.

[paulca]

Can anyone see why this won't work?

https://hobbyking.com/en_us/turnigy-b6-compact-50w-5a-automatic-balance-charger-2-6s-lipoly.html?___store=en_us
and:
https://hobbyking.com/en_us/zippy-flightmax-8000mah-3s1p-30c-lipo-pack.html?___store=en_us

With a plain old DPDT relay switched on the car circuit.

Car circuit present the relay switches the battery over to the charger (and powers the charger)
Car circuit not present the relay switches the battery to power the dashcam.

[paulca]

This is what I'm thinking (link below):

It's difficult to simulate the charger in there so it's just a 3 ohm load, the dash cam a 90 ohm load.

The second terminal on the lower relay would connect the battery + back to the charger.

To toggle the circuit switch the left hand voltage source between 12V and 0V to simulate the car supply going on and off.

The only other thing I need to consider is how the battery and charger would behave with it's balance lead connected to an unpowered charger while power is consumed from the battery.  To disconnect them when the car supply is 0V I would need to do it in a way that would not alter the voltage on them while the car supply is on so transistors are out.  Another 3 (or 4) relays seems unwieldy.

Circuit Simulation