Can I use a battery isolator to top up my car battery?

[douglasheld]

Hello forum,

System Design

I have a Lexus HS250h. This car has a Toyota hybrid system similar to a Prius. The system includes a high voltage traction battery which is used to store energy for vehicle movement, and also a 12V AGM battery which is used to power on the control logic and high voltage relays, and provide utility power for things like lighting and the dashcam. As long as there's about 9-10V available from the AGM battery, the car can start. Otherwise, the 12V circuit needs an external power source (a jump) to turn the car on.

While the car ignition is "on", a DC-DC converter takes power from the HV battery and manages recharging of the AGM battery. So that means, when the car is running there's approximately 13.5V.

I've installed a dashcam. This Blackvue DR750X monitors voltage inputs from both the 12V battery connection and separately, the switched "ignition" signal which usually runs at the AGM battery charging voltage. When the ignition circuit goes to 0V, the dashcam monitors the voltage on the battery circuit and keeps recording motion events or accelerometer events until the battery voltage dips to 11.5V. Then, the dashcam powers off.


Problem Statement

Recently my car was damaged while parked, and this unfortunately happened in a 40 minute period when the dashcam had powered down. I really don't have any control over this design. I can choose the power-off voltage threshold and 11.5 is already the lowest setting. If I hack something to go lower, I run the risk of not being able to start the car, and that's a greater operational risk than losing forensic data.

I don't have any control over the car's design. The way the car was designed, it can sit unused for 4-6 weeks and there is still enough charge available in that AGM battery for the car to start. However with the dashcam added, it's now drawing the battery down to 11.5V within an hour or two of parking.

The idea is: If we have solar energy available, can I use that to safely extend the amount of time the camera can record in parking mode?


Proposed Solution

Use a battery isolator and a small PV panel under the rear window to recharge the AGM battery whenever the car is parked in sunlight. And so this isolator is going to have to be connected to that existing circuit, where the voltage available could be anywhere from a low AGM battery at 11.5V, or the DC-DC converter could be pushing 13.5, maybe 14+V during charging.


Questions

Since the battery isolator's design goal is to connect two batteries of different chemistry... does it actually make sense for me to repurpose that to connect two charging circuits to a single battery?

Is there an off-the shelf solution for my problem?

Can anybody see potential problems with my approach? (Note, the Toyota DC-DC converter is very expensive to replace)

 

[Supervstech]

I'm not sure why you'd want an isolator.
Just a solar trickle charger should do it.
The tiny ones won't provide enough power if your camera is dipping you that fast, ESPECIALLY if you put the panel inside the car.
You are going to need more watts than the camera draws, so find out what that is.
I'd bet you need 15W minimum.
And the panel will need to be on top of the car.
You could likely build a magnet mount like a delivery car would use.

 

[sunshine_eggo]

First, the HS250h uses a battery similar to the Camry, but it is actually a custom case for your model that is one of the most annoying batteries to install. It is a Generation 3 Toyota hybrid.

Second, if you're actually letting the car sit for 4-6 weeks, you're going to kill your hybrid battery. Letting them sit for 4-6 weeks at a time is like shooting yourself in the head at point blank range and hoping you'll miss. They are meant to be driven, the cost-benefit analysis relies on actually driving the car to get the benefits.

Third, they are not designed to sit for 4-6 weeks. Toyota dealer practices require that any hybrid on the lot for more than 30 days must be engaged in ready mode for 30 minutes to ensure the 12V stays charged and the HV battery is lightly cycled. 4-6 weeks of sitting requires you to deactivate the smart access system with push-button start in advance (U.S. manual page 584).

Clearly, @Supervstech gets to the point faster than I do, and I concur. I am dubious a 15W panel is sufficient. I would start with twice that.

 

[chrisski]

Do you know the size of this battery?

The car batteries I deal with are maybe 80 ah, and this seems like a lot of power to go through for a dashcam.

That’s a lot of power to go through in an hour. Going down to 11,5 VDC on a fell healthy battery in an hour points to something draining a lot of power.

 

[sunshine_eggo]

Do you know the size of this battery?

The car batteries I deal with are maybe 80 ah, and this seems like a lot of power to go through for a dashcam.

That’s a lot of power to go through in an hour. Going down to 11,5 VDC on a fell healthy battery in an hour points to something draining a lot of power.

Toyota hybrid charging systems are marginal, so the battery is likely at a mid level SoC at best. Since the 12V never does anything but power the 12V system for about 2 seconds before the HV battery relays are closed, they can last a crazy long time even with poor charging habits. This battery is about 60Ah. I routinely run into 8-10 year old hybrids on their original 12V.

Meant to mention it to the OP... I vigorously recommend the 12V be charged at 6.0A to 14.4V and held there for at least 2 hours.

 

[douglasheld]

Toyota hybrid charging systems are marginal, so the battery is likely at a mid level SoC at best. Since the 12V never does anything but power the 12V system for about 2 seconds before the HV battery relays are closed, they can last a crazy long time even with poor charging habits. This battery is about 60Ah. I routinely run into 8-10 year old hybrids on their original 12V.

Meant to mention it to the OP... I vigorously recommend the 12V be charged at 6.0A to 14.4V and held there for at least 2 hours.

Thanks very much. Your observation matches my suspicion exactly.

The car is from January 2012 and I received it earlier this year with its original battery. I replaced it with a new Lexus battery, the spec being S55D23R. It is Panasonic, I think 51Ah.

 

[douglasheld]

if you're actually letting the car sit for 4-6 weeks

I can stop you there - I'm not

I drive it at least once a week to the grocery store, but it is true this car doesn't get a lot of use. It is 12 years old and only has 27k miles.

 

[douglasheld]

I'm not sure why you'd want an isolator.
Just a solar trickle charger should do it.
The tiny ones won't provide enough power if your camera is dipping you that fast, ESPECIALLY if you put the panel inside the car.
You are going to need more watts than the camera draws, so find out what that is.
I'd bet you need 15W minimum.
And the panel will need to be on top of the car.
You could likely build a magnet mount like a delivery car would use.

Thank you very much!
This dashcam, the specification says "Parking Mode (WiFi Off / GPS Off / 2CH) : 285mA / hour"
Now leaving aside what an "Ampere per hour" may be I guess that is 0.285 * 12V = 3W.

If we have a low SOC battery in the system, I am not sure I agree that my goal should be to replenish the camera's power requirement.

Looking at @sunshine_eggo 's recommendation,

I vigorously recommend the 12V be charged at 6.0A to 14.4V and held there for at least 2 hours

Thank you, I will investigate how to charge up this battery.* Once the car battery gets fully charged up, maybe the whole system will function more nicely. Or maybe it will need a boost a couple times a year.

I left out a piece of information that now seems relevant: When I first had this dashcam installed, the installer didn't wire it according to the instructions, and it ran the brand new AGM battery down to somewhere around 9-10V. Maybe this battery has simply never recovered its state of charge from that mishap?

* without blowing myself up

 

[douglasheld]

I'm not sure why you'd want an isolator.
Just a solar trickle charger should do it.

Thank you - my main reason for thinking an isolator might be needed is the fact the PV input would still be there while the car is running. And I don't know all the information about the requirements of the car's onboard charging system. Did Toyota design it with consideration for another 13-14V power source on the battery circuit? Probably not! I don't want to damage something expensive.

 

[sunshine_eggo]

I can stop you there - I'm not

I drive it at least once a week to the grocery store, but it is true this car doesn't get a lot of use. It is 12 years old and only has 27k miles.

I'll be blunt. You're lucky the HV battery is still operational, and it's a ticking time bomb. At least 3 drives per week > 10 miles plus additional driving that yields 500 miles/month average is the minimum amount needed to maintain cell balance in an old battery (both age and miles affect it). There are 204 individual cells in the battery, and they all lose charge at a different rate. Regular driving helps keep them in balance (balance = each cell at the same state of charge).

A grid charger designed to balance the NiMH cells could prevent failure due to imbalanced cells. This gent:

http://www.solarvan.co.uk/

Is in the UK and is a whiz with hybrids. I don't know that he offers a product, but he can likely steer you in the right direction.

Thank you - my main reason for thinking an isolator might be needed is the fact the PV input would still be there while the car is running. And I don't know all the information about the requirements of the car's onboard charging system. Did Toyota design it with consideration for another 13-14V power source on the battery circuit? Probably not! I don't want to damage something expensive.

Toyota's charging system doesn't care. Just ensure the supplemental charging system is limited to 14.4V.

 

[sunshine_eggo]

Thank you, I will investigate how to charge up this battery.* Once the car battery gets fully charged up, maybe the whole system will function more nicely. Or maybe it will need a boost a couple times a year.

I left out a piece of information that now seems relevant: When I first had this dashcam installed, the installer didn't wire it according to the instructions, and it ran the brand new AGM battery down to somewhere around 9-10V. Maybe this battery has simply never recovered its state of charge from that mishap?

* without blowing myself up

Missed this bit. Yes. A discharge like that can put you behind the curve especially with as little driving as you do, and this would explain the rapid depletion @ 11.5V after parking. You have likely never recovered to a full state of charge, and the battery has sustained damage with permanently reduced total capacity.

Nothing complicated about charging this battery. Any 12V lead-acid charger is better than none. One that charges at 4-6A and has an "AGM" mode is probably the best.

Essentially zero risk of blowing yourself up. I've spent thousands of hours abusing lead acid batteries and never even came close to a catastrophic situation.

 

[Supervstech]

As winter approaches you have GOT to get a new battery in there.
If 280mW can drain down in an hour, you need a new battery.
And, yes,a full charge is a requirement on them.
In winter, a low state of charge can freeze thw battery.
Agm is less susceptible to it, but I wouldn't wanna chance it.
Get a charger on there that has an agm setting and plug it in overnight.
If it hits 13.08V and doesn't drop below 12.6 with the charger off you should be ok.
I would want an ammeter on the battery to measure true discharge.

 

[douglasheld]

Toyota hybrid charging systems are marginal, so the battery is likely at a mid level SoC at best

I'm almost sure now that I've brought this problem upon myself, ironically by trying to maximize the amount of dashcam parking recording time.

By setting the low-voltage cut-off to the lowest value, my AGM battery is always at a low SOC and never recovers. I'll set it instead to the highest cut-off option.

 

[douglasheld]

I'm almost sure now that I've brought this problem upon myself, ironically by trying to maximize the amount of dashcam parking recording time.

By setting the low-voltage cut-off to the lowest value, my AGM battery is always at a low SOC and never recovers. I'll set it instead to the highest cut-off option.

I would appreciate if anybody could reference a calculator to work out whether this is going to be better in terms of recording time. My car got scratched again last night!

Old scheme:
Camera discharged the battery down to 11.8V (30% SOC)
Car's DC/DC converter charged the battery back up, at whatever current rate it could support, during the ±40min/week or so of driving time.
Resulting SOC allowed ±40 minutes of discharge back to the 30% SOC level.

New scheme:
Camera will discharge the battery down to 12.5V (80% SOC)
The car's DC/DC converter will charge and maintain the battery, only while the car is running, during the ±40 min/week that it gets used.
The resulting SOC is probably not going to get up to 100%, but will there be more Coulombs in there available for use, or fewer?

I'm guessing it will be about the same, but I will have a slightly healthier battery. Maybe I shouldn't give up on my original idea, to install a solar trickle charger to replenish that battery so something works on it even when the car isn't running.

 

[sunshine_eggo]

285mA is about 10-20X the normal load of the parked car.

Your ±40 minutes of charging every week is ALL you have to work with. That gives you X Wh. It (mostly) doesn't matter weather it's at a high SoC or a low SoC. Let's say that the car can charge the battery at 10A for 40 minutes.

That's 10A * 40min/60min/hr = 6.7Ah.

You can run your camera 6.7Ah/.285A = 23 total hours for the WEEK, and this is very idealized. There are a lot of losses between the converter and the 12V battery.

Buy yourself a lithium jump starter, so you'll never be stranded. Any cheap one will do. The hybrid's 12V battery provides very little current, so you don't need a beast.

Get the largest PV panel (example, not a recommendation) you can easily fit in the back window and install it such that the roof and pillars don't shade it. Lay it flat when driving and prop it up when parked.

Run it to a PWM charge controller attached to the battery. Charge to 14.4V and float at 13.8V.

You'll need to routinely park in such a way as to optimize solar exposure assuming a place like this exists in the UK.

No isolator needed.

 

[chrisski]

Get the largest PV panel (example, not a recommendation) you can easily fit in the back window and install it such that the roof and pillars don't shade it. Lay it flat when driving and prop it up when parked.

I did not see if the OP was parked indoors or outdoors. If parked inside like a garage, would this panel work?

I have used a solar panel to fight a small parasitic draw. I found that my windshield filtered 2/3 the usable energy. Since I was just fighting 50 ma draw that was killing my battery when parked for a week at the airport, the panel was fine to keep the charge.

I did not need a dash cam so In my case an even better solution was I disconnected the battery when parked long periods of time.

 

[douglasheld]

I did not see if the OP was parked indoors or outdoors. If parked inside like a garage, would this panel work?

Thanks, I park outdoors. The panel would not work inside

 

[douglasheld]

285mA is about 10-20X the normal load of the parked car.

Your ±40 minutes of charging every week is ALL you have to work with. That gives you X Wh. It (mostly) doesn't matter weather it's at a high SoC or a low SoC. Let's say that the car can charge the battery at 10A for 40 minutes.

That's 10A * 40min/60min/hr = 6.7Ah.

You can run your camera 6.7Ah/.285A = 23 total hours for the WEEK, and this is very idealized. There are a lot of losses between the converter and the 12V battery.

Buy yourself a lithium jump starter, so you'll never be stranded. Any cheap one will do. The hybrid's 12V battery provides very little current, so you don't need a beast.

Get the largest PV panel (example, not a recommendation) you can easily fit in the back window and install it such that the roof and pillars don't shade it. Lay it flat when driving and prop it up when parked.

Run it to a PWM charge controller attached to the battery. Charge to 14.4V and float at 13.8V.

You'll need to routinely park in such a way as to optimize solar exposure assuming a place like this exists in the UK.

No isolator needed.

Thank you, this makes sense.

There are a few obstructions in the rear shelf but the good news is, that window sits at a very low angle. I have two rectangular spaces, about 46cm x 25cm, to work with. That's a little too narrow for a typical 10W square panel. I might need something custom.

If I buy some individual monocrystals and commit to soldering them together, I can find a number of options for my usable area:

• 165mm x 135mm cells, I can fit 3 of them (rated 6W each) into each of the two spaces. Total 1337 cm^2.
• 170mm x 86mm cells, I can fit five of them (rated 1W each) into each of the two spaces. Total 1462 cm^2.
• 107mm x 61mm cell s, I can fit 15 of them (rated 1W? each) into each of the two spaces. Total 1958 cm^2
• 50mm x 50mm monocrystals (rated 1W???) then I can fit 28 in each of the two spaces. Total 1400 cm^2.

(It goes without saying I don't trust the power rating, so I am going instead with the monocrystalline formulation and just unit area.)

New Question 1: If I buy the individual components like that, and solder them into a rectangular structure, what is a typical product people use to mount these into a nice array? I couldn't find an off-the-shelf DIY material but there must be a market for it. As it's indoors I don't need an aluminum frame or protective glass, etc. But something like an adhesive backing or conductor support would be nice - as well as a thermally supportive material. I imagine the crystals don't work as well if they get too hot.

New Question 2: Is there a resource where I can shop for prefabricated monocrystalline panels, sorted by size?

 

[douglasheld]

285mA is about 10-20X the normal load of the parked car.

Your ±40 minutes of charging every week is ALL you have to work with. That gives you X Wh. It (mostly) doesn't matter weather it's at a high SoC or a low SoC. Let's say that the car can charge the battery at 10A for 40 minutes.

That's 10A * 40min/60min/hr = 6.7Ah.

You can run your camera 6.7Ah/.285A = 23 total hours for the WEEK, and this is very idealized. There are a lot of losses between the converter and the 12V battery.

Buy yourself a lithium jump starter, so you'll never be stranded. Any cheap one will do. The hybrid's 12V battery provides very little current, so you don't need a beast.

Get the largest PV panel (example, not a recommendation) you can easily fit in the back window and install it such that the roof and pillars don't shade it. Lay it flat when driving and prop it up when parked.

Run it to a PWM charge controller attached to the battery. Charge to 14.4V and float at 13.8V.

You'll need to routinely park in such a way as to optimize solar exposure assuming a place like this exists in the UK.

No isolator needed.

By the way, @sunshine_eggo your comments are incredibly detailed and helpful. I really appreciate that.

 

[SamDeleted]

@douglasheld , maybe I'm missing something here , but if you're concerned about messing with the existing DC system , why not just chuck a deep cycle battery in the boot (trunk) and run the camera off that