Have a geeky question here and if someone does electronics they may be able to help.

[NightShade]

Ok so here it is. It's time to replace the battery in my truck and I want to do something a little different. There are video's on Youtube where capacitors are used for the start power. I know it will work but they also have an issue where they discharge and have issues when an alarm is in play due to constant discharge. The way around that is to setup a hybrid system where the capacitor bank is tied in with a battery, this way the caps can provide the starting power but the battery provides the power to keep everything running. So my thought is to use a deep cycle battery that can handle being drained down a bit more over time.

The issue that comes into play is the capacitors can take a fast charge and discharge rate and likely pull too much from a battery. So what do I need to do to limit the charge and discharge rate of the battery? Most of the people I have seen do this just use a cheap 20 dollar 12V battery and ignore it which is an option but if I am working on my truck and have the radio on that will be an issue, a deep cycle battery however will be able to supply power for a while longer and since my plan is to mount it in the bed of my truck size is not an issue there. The caps will be able to do the starting and other high power draws without issue. But drawing big loads like starting off a deep cycle battery will end up killing it fast so I want to limit the current that can be pushed into or pulled out of the battery.

Using a combination of some capacitors as well as a deep cycle battery should allow me to have both the ability to start the vehicle as well as draw power over time and have a long lasting battery if I can limit the current draw on the deep cycle. I am just having a hard time finding the information on what I need to do to make this work and not limit voltage. I am hoping to figure something fairly simple out as well and right now my google fu is off and so is my brain.

 

[MacFromOK]

I don't think this will work like you expect. There's a reason caps haven't replaced batteries in heavy-duty/high-amp applications.

Just curious, why are you wanting to do this? It will likely be more expensive and less dependable than a decent battery.

 

[MacFromOK]

To answer your question though... you could use a relay and disconnect the battery from the starter motor while cranking from the caps.

A gorilla resistor would also do the job, but you'd lose a good bit of power through the heat it would generate.

FWIW, a charge pump to boost voltage into the caps (similar to a camera flash circuit) would give more available cranking amps.

Also... heavily discharging caps can greatly shorten their lifespan.

 

[NightShade]

I don't think this will work like you expect. There's a reason caps haven't replaced batteries in heavy-duty/high-amp applications.

Just curious, why are you wanting to do this? It will likely be more expensive and less dependable than a decent battery.

The large caps actually can provide enough cranking power. They even make booster packs now that are designed to do exactly this. https://amzn.to/377sIG9 Just about every retailer has one of the packs available.

Part of the reason is I have been having issues with the battery running down. It's harder on a regular battery than a deep cycle but swapping in a deep cycle has it's own issues since they don't handle providing cranking power well which can shorten the battery life. Having a built in "jumper pack" while a little more costly to begin with in combination with a deep cycle battery may cost a little more but should outlive it's regular battery component and the replacement of the deep cycle battery is cheaper than a regular battery so over the lifespan of the whole system it should actually be cheaper. The one issue is keeping the draw from the battery from being too much when starting or between cranking if the first try doesn't work.

These caps https://amzn.to/37aJWTd are actually being put into ready made replacement packs for some vehicles as well. https://www.maxwell.com/products/esm/esm-ultra-31-1800 But they have an issue without how dense the energy storage is but they are nearly at a point to put lithium ion batteries under.

If it's not something I can figure out I will probably end up with a pair of group 27 or 31 batteries in parallel or a 4D or possibly an 8D

The issue with a relay or disconnect is that when reconnecting between cranking operations the capacitors could suck too much juice if what I read is correct. I thought about a 200w .1 or .5 ohm resistor as well but I am wondering if there is a better way from my understanding doesn't the resistor also cut the voltage after it as well?

 

[MacFromOK]

I thought about a 200w .1 or .5 ohm resistor as well but I am wondering if there is a better way from my understanding doesn't the resistor also cut the voltage after it as well?

There is a voltage drop, but it should be small if you only use enough resistance to prevent battery damage.

It will put out a lot of heat though. Basically, what you're trying to do is (briefly) protect the battery from a direct short to ground. :/

 

[NightShade]

There is a voltage drop, but it should be small if you only use enough resistance to prevent battery damage.

It will put out a lot of heat though. Basically, what you're trying to do is (briefly) protect the battery from a direct short to ground. :/

Right, so something like https://www.newark.com/tt-electronics-welwyn/wh200-r10ji/through-hole-wirewound-resistor/dp/46R5085 would be a decent option?

 

[MacFromOK]

I think 200W will be kinda wimpy.

 

[Snattlerake]

If you are simply trying to maintain a good battery for a longer period of time why not get a second battery in parallel with a heavy duty alternator like an ambulance?

Or am I missing the whole point of this?

 

[NightShade]

I think 200W will be kinda wimpy.

Ok so 400 maybe?

If you are simply trying to maintain a good battery for a longer period of time why not get a second battery in parallel with a heavy duty alternator like an ambulance?

Or am I missing the whole point of this?

I have a high amp alternator https://www.mechman.com/alternators...240-amp-alternator-for-3-7-4-7-chrysler-late/ And I could go dual battery as well, but I am just working my options. The capacitors can literally take thousands of complete discharge cycles without issue and can easily last ten to twenty years while a set of batteries used to start are usually replaced around the 5 year mark and continually lose their ability to store energy to sulfates. I could use an oversized deep cycle as well which should have the ability to last longer. Ideally I would go with a LifePO4 battery however they do not do well with cold temperatures like a vehicle will often deal with. A lot of this is figuring the pro's and cons of everything to find something that will last a long time as well as provide what I need at a decent price point. The use of a group 27 or 31 deep cycle along with a capacitor bank would give me the ability to have the cranking power needed with a fairly long battery life if too large of a load is not applied to the battery in one shot hence the addition of something to limit the current draw. I have not decided yet if any one way is going to be best but I hate the thought of having to buy vehicle batteries at 130 to 150 every five years if I can spend 200 now and replace the battery every 10 years at 80.

 

[Dumpstick]

You can use a deep cycle battery as a 'house' battery to run radios etc.
The regular battery is used as a start battery. There are dual battery isolator switches set up for just this purpose, you can get them at any rv shop.
The switch charges the start battery first, after the engine starts, the charges the 'house' battery.

Or, are you wanting to use capacitors just to see if you can?

Edit to add: link for isolator diagram
https://ricardolevinsmorales.com/battery-isolator-wiring-schematic/