alternator to charge house battery

[Van Go]

I have a 2002 Toyota Tacoma 4-banger that I'm going to camp in for about 3 weeks. I need to charge an Optima house battery (blue top) from my stock alternator. I need the battery to run my CPAP machine (off a pure sine inverter). I need the simplest, cheapest way to do this. Would like to use the stock alternator. So, I think I run #6/8 copper from the alternator + pole to  some kind of switch. Manual switch  preferable since this is a short term hook up and I will probably leave the wiring in place. I would prefer to run copper back to the alternator ground. What brand parts should I get. Please, don't scold me. I have searched and found info but just need a simple, straitforward explanation of how to do this. BTW my machine runs on 24 volts (I guess in converts 120VAC) and lists draw as 1.25 A (FG) and 3.75 A (Sys)----I really don't know what the draw in amp hours will be. I will run it 8 hours a day.
I will have a 120 volt charger with me in case I have 120V at a campground, so I can top off the battery when possible. 
Please advise if this is not feasible. 
I don't have much $ to spend, but don't want to use cheapo parts. TIA.

 

[highdesertranger]

do you realize that it takes many hours to recharge a depleted battery from an alternator. also it should be recharged daily and sitting and idling won't do it. don't get me wrong charging from the alternator is an important piece of the charging puzzle but to rely on it as your sole recharging source usually doesn't cut it. highdesertranger

 

[Van Go]

yes, I understand. it is a 55 amp hour battery. I'm estimating I will run it down 20 % each night. (I don't know how to convert 1.25 amps at 24 volts to x-amps at 12 volts.) If I drive an hour or two a day could I at least keep it at 80% charge for 3 weeks. I don't think that would kill this battery. I also may be able to fully charge it at a campground occasionally with a 110volt charger .

 

[Van Go]

I think the stock alternator is 70 amp.

 

[tx2sturgis]

Simplest and cheapest will be a manual battery switch that creates a parallel connection to your starting battery. 

Another simple and cheap option is a battery isolator.

Both have pros and cons. 

Is your Optima battery in the passenger floorboard, or in the bed of the truck?

As HDR said, unless you drive a lot, then you may not be able to bring the auxiliary battery back to full charge.

What you might want to look at is one of the CPAP machines made for campers and travelers:

http://mytranscend.com/products/power-accessories/?device=2084

They are efficient, battery powered, and they have a solar charger option.

 

[Van Go]

The battery will be in the bed of the truck. So it sounds like you're talking several hours a day driving to get the battery to full charge. Would it hurt the battery to get to say 80% charge daily for a few weeks?
I don't have much $ to spend. This is just a 3 week trip. And may be the last one I ever make. So I can't invest in a new CPAP. Just need to make it work for 3 weeks.

 

[Van Go]

I use the 80% figure because I have read that you can get there pretty quickly, but the last 20% takes time.

 

[highdesertranger]

1.25 amps at 24v is 2.5 amps at 12v. you double it. so 2.5AH x 8 hours = 20AH. you have a 55ah battery of which you have 27.5AH of usable power. taking 20AH away from that you will have a very depleted battery everyday. you will need to drive 6-8 hours a day to recharge from your alternator. also if your battery is not 100% recharged everyday you will have less power everyday. going by your 80% on the second day you will only have 22AH available by the third day you will be discharged below 50%. by the fourth day you will not make it through the night. highdesertranger

 

[highdesertranger]

after reading post #6. I would say get more/bigger batteries. highdesertranger

 

[Van Go]

Thanks HDR. I can see that this will not work with what I've got. I guess I need to look for campgrounds with 120v hook-ups. Thanks for your help.

 

[John61CT]

For battery longevity you should get the battery to 100% Full at least a couple times a week. A good shore charger at a campground is lower up-front than solar, but ongoing more expensive.

The alternator will do a great job between 50% (lowest you should go) and 80%.

You need to actually measure AH getting into the batt to see how long a trip that takes, maybe under an hour actual driving. Just idling with a stock setup, who knows.

A proper battery monitor is expensive, well over $100.

But $20-30 will get a cheap eBay unit like "Watts Up" that can help, once you figure out how to use it.

 

[MrAlvinDude]

A few thoughts. 

---

The simplest way to do your setup, would be to use battery clamps, like those on jumper cables. When you hook up your house/auxiliary battery. 

To switch on the house battery, you attach the jumper cable, and to switch it off, you detach the jumper cable. 

As this might be a one time, 3 week event, this solution might work just fine for you. 


Any refinement, like more permanent  wirering, adding a switch, or a manually controlled solenoid, can be considered upgrades that will improve the ease of aconnecting and disconnecting the house battery to the alternator charge system.

--- 

It is possible to use a voltage booster to maximize the charge speed of a battery, while your engine is only running idle. 

Here is an example of an inexpensive but very dependable voltage booster. 
https://www.ebay.com/itm/LTC3780-Au...p-up-Down-12V-24V-Solar-Charging/172419671126

This board uses components that are specifically developed to be used in the automotive industry, so it is very well suited to be used for battery charging, from a 10V to 16V volt power source which is what is typical in cars and such. 


This boards output voltage level is adjustable, so you could have one board you use to optimize charging speeds of your house/auxiliary battery, while your engine is only running idle. 

And another board that can convert the 12V from the battery, to the 24V that you CPAP machine needs. 

I know that you already have units so you can create 120V and then reduce that down to 24V, but this conversion will take more loss of power, than to just convert from 12V to 24V. 

---

Another way to create 24V, is to have two 12V house batteries. 

When you charge them, you connect them in parallel, and use the 12V system in the car to charge them. 
When you need to run the CPAP machine, you connect the two batteries in series, and you will then have 24V available for the CPAP machine. 

This setup minimizes the energy loss that any voltage converters or inverters would otherwise cause. 

---

If you end up choosing to try out a battery solution, then I would recommend that you get yourself a multimeter (or any meter that can measure voltages in the 12V range) 

This way, you can yourself monitor the status of your house/auxiliary battery. 

You will want to be able to measure voltages between 11.8V and 14.2V. As this is the range that your typical 12V car batty operates within.  

--- 

I happen to have a 55Ah 12V battery, and one of those voltage boosters in my workshop, so if you like, I could set up a test, over the weekend, to see how long it would take to charge a typical car battery between 50% and 80% 

Or did you already decide to go with the shore power option?

 

[highdesertranger]

Alvin that board will not work, while the volts are fine the amps are meager. the recommended amp out put is 4amps at 12 volts. that will not charge a battery, maintain it yes but charge it not really . but do your test and report back make sure you deplete your battery first. highdesertranger

 

[Van Go]

Thank you all for your input. I think I will look for campgrounds with power. But, I will take the battery, inverter, and shore charger with me. I have made it 2 nights with a full battery before. So, I am not sure if the CPAP pulls as much amperage as listed. I can survive one night without it if I have to. Again, thanks for all the effort put forth. You guys are great.

 

[MrAlvinDude]

Hi VanGo,
your decided approach sounds like the most straight forward (and cheapest) solution.


Hi  highdesertranger.
I already have one of those boards, and from my preliminary tests it does indeed show that without additional heat sinks attached, the board does get too hot if it is set to more than 4 amps out.  

However, by adding additional metal, or a fan, or a regular heat sink, then it can actually give closer to 10 amp out.  
When used in a car, then the (total watt) limit is however determined by the on-board input fuse of 10 amp.

So if the alternator only gives for example 12V when the engine is idling, then the max watt  capacity is  12V x 10A = 120 Watt => 120W / 14.4V = (max) 8.3A out. 



And the output voltage is very much adjustable. It is in the rage of 1V to 32V. So it is quite simple to set it to, for example, 14.4V or 14.6V out.


As the  number of amps that can possibly be pumped into a battery is a function of the voltage difference of the battery and the power source, in relation to the internal resistance of the battery, my test would mainly show at what percentage of charge of the battery, that it would no longer be possible to push more than for example 5 amp or 2 amp into the battery.

But as VanGo has already decided on a solution without this board, my extended testing is not likely to happen this weekend, but will be done at a later date. 

When I have more specific numbers to share, I will however make a new thread, with test results and a few pictures.

 

[John61CT]

A proper DC-DC charger like CTEK's, or one of Sterling's batt2batt units is initially expensive, but will do the job right and last many bank replacements, maybe multiple vehicles.

 

[John TF]

Lots of bad info , and some good here .
Without custom building , the most efficient battery / money spent , is still FLA - Flooded Lead Acid battery .
And generally speaking start at single cell is 2.1 volts for large house / commercial use , for RVers use the best [ common ] use is 6v - 3 cell .
It is common for having deep cycle battery system = the number of cells to build your needed voltage , is going to be the above 6v battery [ there are some quality 12v deep-cycle , but not as common ] so one of the terms for these is golf cart batteries .
My last battery bank was 4 Trojan L16's , 2 pair is series for 12v and the series pair in parallel for more capacity .
Used with a good quality inverter & only solar to charge , this system last over 13 yrs. running 30' Airstream airconditioning , everything .
And for this use , deep cycle you need a REAL deep cycle battery , not anything that says marine / RV - these are a combination of 2 types of manufacture - starting & deep cycle , so performance of either is compromised .
Any lead acid battery from the point of manufacture is starting to discharge & sulfate - the more it is discharged the higher the rate of sulfating , so charging as soon as possible / constant , as high as possible current [ relative the to the C rating of battery bank ] .
There are reasons for using different specialty batteries AGM , GEL etc. , but at a higher cost , maybe double or more ?
So unless your flying a plane upside down , or in a non ventilated area [ easily over come ] , use FLA batteries .
As for charging from engine , for non emergency / longterm , you should use a battery isolator for several reasons , 1st to not discharge the starting battery , next is the charging system can sense the fully charged starting battery , and not hardly charge the secondary battery , which brings up the 3rd - charging current is supplied mostly by engine RPM and typically a min. of 1,300 RPM ++

 

[frater secessus]

...I at least keep it at 80% charge for 3 weeks. I don't think that would kill this battery.

That is a classic method of battery murder.  

Running a battery at partial state of charge for weeks without damage will require something exotic like lithium or carbon-foam.  Bring all your money when you go to pick up either one.

 

[John61CT]

Some people don't mind replacing their batts 4-6 times as frequently

 

[666JTK666]

Hi folks,

I know that this topic is pretty much solved for the OP But I thought I would put my 2c in because I did quite a bit of research on this.

equipment needed.
1. idle kickup device this when parked would auto idle up to an adjustable rpm around 1200.
2. a good heavy duty voltage regulator like the balmer MC614
3. an alternator mod or stock for external regulation.
4. inho sterling make the best dc-dc battery chargers.
5. batterys monitering system.

This would all be wonderful if it was not for the LOOOOOOOOOOOOOOOng absorption stage for fla batterys to get to a full charge.burn a 
I think this would be a good system if it were combined with solar for the absorption stage. 

Still it's going to burn a lot of fuel.