How to charge my 36v 15ah SLA battery from the car?

[kmxtornado]

Please recommend me a power inverter. I believe I'm looking for a power inverter to be able to charge my 36v 15ah SLA battery from the car.
1. Bestek 1000w?
2. Power Bright 900W Part number: PW900-12
3. Power Bright 750W Part number: XR750-12 (doesn't look as cool as the one above. Haha).
4. Schumacher Electric 750W Part number: XI75DU

I HAVE AN INVERTER BUT IT DOESN'T SEEM TO DO THE JOB:
The problem I am having is that the current one I'm using ($13, 150W 12V DC to 110V AC) doesn't charge the battery all the way. It stops prematurely and the light on my charger shows that it's fully charged but in fact it is far from fully charged. I figure I need a new inverter or one of a different spec. When I charged my 24v battery, it worked just fine. Not so with the 36v battery. The scooter it's powering has a 500W motor. Not sure if that piece of info is relevant.

PURPOSE OF THE INVERTER:
My goal is to purchase something reliable, effective and safe. I don't mind spending the money. I'd like something around $70, if that's reasonable. The time it takes to fully charge the battery isn't that big of a concern, but it would be good to know the sacrifices I'd perhaps have to make given my budget. I typically don't ride my scooter until it's completely empty (or close to it since it's an SLA), so I'm typically okay with whatever I get by charging it for an hour in the car. Needs to have a cigarette lighter adapter which I understand I can purchase separately as most of these inverters come only with alligator clips.

SIDE QUESTIONS:
1. Is it important that I charge my battery while the car is moving so that the alternator is continuously charging the car's battery? Or is it okay to charge my scooter battery while the car is not moving? Don't want to drain out the car battery and get stranded. Not sure if I'm being paranoid or this is a valid concern. I'm reading that I should start the car for 10-15 minutes every hour. Comments?
2. Do I get one with a "true sine wave" or "modified sine wave" or is that way too technical and not even worth mentioning?

I'm currently reading up and educating myself on power inverter buying guides. Any expert help would be greatly appreciated. Thanks in advance!

WHAT I THINK I KNOW SO FAR: Please correct me if I'm wrong.
- So far I'm gathering that I need 36v x 15ah = 540w inverter. Maybe that's my problem. My current one only gives me 150w so maybe it's not charging any more once it gets the battery to a particular wattage, 150w. Is that how that works?
- I'm reading that I should get something that's 20% higher than what I actually need, making that 650w. 650w inverters are either not available or not very common. So perhaps a 700w or 750w is that I should be getting.
- I need a version that supplies "continuous power," not "peak surge."

 

[wesnewell]

I can only assume you have a 36V charger that needs to plug in to 110VAC to operate and charge your 36VDC battery. The 12V inverter needs to output the watts required by your battery charger. That info should be on a label on the charger. it may be listed as amps, so you'll need to convert that to watts if that's how the inverter is spec'd. Any of the inverters you listed should work fine as long as you don't have a super big 36V charger. You may have to hook it directly to the battery and not use the lighter adaptor as they are usually fused at ~240W with a 20A fuse.

 

[dogman dan]

Yes you will need to run the car while you charge. If you don't, you better have a stick and be parked on the hill. Because of this, you might want to consider a very small generator. Yes, it will cost more than $70, but you can get them as cheap as about $150 at harbor freight or someplace like that.

I mean really, running a whole car to charge a tiny battery sucks, except as an emergency thing. I'm assuming you want to go spend the day, or camp in places where there is no plug. I do this at times too, and use my 700w Honda generator.

A generator has both a 110v plug, but a 12v outlet that will jump your car. Can be handy! They also rock when you have a power outage at the house for some reason. Even if not enough power to run a coffee maker, you can at least get a few lights working. Get the tv on to see what's going on etc.

If you must do the inverter thing, then a 300w one should do the trick. That can work fine if you are driving to another spot anyway, but it takes a damn long drive get a full charge. Like 200 miles.

 

[Russell]

Due to the poor power factor of most chargers it's a good idea to choose an inverter with at least twice the continuous watt rating as the charger.

-R

 

[kmxtornado]

Last night I read that the cigarette lighter plug in the car is only rated for 150w - 300w depending on the car. Sometimes it can go up to 400w. But 700w for my need is way up there, so unless I go with the generator idea from Dogman, I'm thinking I might want to just hire a radio guy to hardwire an inverter directly to my car battery and use an existing (or new) firewall to get the fat cables through to the inside of my car. Perhaps hide the inverter under the seat or something.

Yes you will need to run the car while you charge. If you don't, you better have a stick and be parked on the hill. Because of this, you might want to consider a very small generator. Yes, it will cost more than $70, but you can get them as cheap as about $150 at harbor freight or someplace like that.

I mean really, running a whole car to charge a tiny battery sucks, except as an emergency thing. I'm assuming you want to go spend the day, or camp in places where there is no plug. I do this at times too, and use my 700w Honda generator.

A generator has both a 110v plug, but a 12v outlet that will jump your car. Can be handy! They also rock when you have a power outage at the house for some reason. Even if not enough power to run a coffee maker, you can at least get a few lights working. Get the tv on to see what's going on etc.

If you must do the inverter thing, then a 300w one should do the trick. That can work fine if you are driving to another spot anyway, but it takes a damn long drive get a full charge. Like 200 miles.

Thanks for your response. I'm strongly considering a generator now. You bring up some good points about other uses for it apart from my wanting to charge up a small battery for a scooter.

Due to the poor power factor of most chargers it's a good idea to choose an inverter with at least twice the continuous watt rating as the charger.

-R

I also understand that I should expect an initial surge that might be more than the overall wattage needed. But that should only be to operate a piece of equipment for those going camping or something.

In my case of just charging a battery, can I get away with just 20% more than the expected full wattage? That brings up another question. If I only use up half the scooter's 650w of power during my rides, can I technically just buy a 300w inverter and expect that it'll charge the battery completely? or does it totally not work that way? layman's logic. I really have no clue about this stuff. Enlighten me. I'm all ears.

 

[kmxtornado]

I have my eye on the Duracell Power Pack Pro, 600w. Problem is that it's 600w, but maybe that's okay being that I'm not planning on running any equipment off of it necessarily. It's main purpose is literally to just charge my 36v 15ah scooter battery.

I like that this one is not gas powered. No choke, no need for ventilation, etc. It's literally like an external battery with a bunch of other cool functions on it from a little LED light, to be able to jump start a car. It even has a built in compressor to fill up car tires. This might be the thing I'm looking for. Any thoughts on why I should be worried or why this might not be what I'm expecting?

Definitely a lot more expensive than $70, but with all the other functionality and possible uses, it seems like a good value at $175. Cheaper than paying someone to hardwire an inverter and risking that they know less about what they're doing than I am. Would be worried that they don't use the correct gauge wire and size of inline fuse even though they should. Another advantage of this portable Duracell unit is that I can transfer it to another car easily. My car is over 10 years old already.

Middleground option would be to simply buy another backup battery. Wouldn't have the flexibility of all the other functions. Pretty much sounds like I'm going with the Duracell product unless anyone has any startling warnings.

 

[kmxtornado]

Upon further reading, the Duracell Power Pack 600 although called the 600, in fact only provides an output of 480w of continuous power. That's less than the 528w requirement before taking the 20% hike into consideration. Thoughts? Still better than the 150w small $20 cigarette lighter plug-in one I have I suppose.

This thing runs on a 12v SLA battery. Is there something wrong? A single 12v battery is charging three 12v batteries? Errrr....wha?

 

[wesnewell]

If you want to do it cheap, just parallel your scooter batteries and hook them up to the car battery with jumper cables. When charged, put them pack in series.

 

[Gregory]

This thing runs on a 12v SLA battery. Is there something wrong? A single 12v battery is charging three 12v batteries? Errrr....wha?

It says it has a 15Ah or 18Ah SLA battery inside.
You'll be able to charge 3 batteries with a third of that into each. Less maybe 20% losses. So 4Ah -5Ah into each of your three batteries.
It can be increased with an external SLA battery.
Doesn't seem like a great option to me.

For cheap and easy I'd try a CCCV Boost DC-DC converter to go from 12V to 42V ish either running off the car (any car stereo or 4x4 installer can fit a fuse and thick power wire for you and a low V battery buzzer), or a spare 50Ah battery.
Or the generator idea.

 

[Gregory]

If you want to do it cheap, just parallel your scooter batteries and hook them up to the car battery with jumper cables. When charged, put them pack in series.

I wouldn't recommend that. With no current limiting you could be hitting the batteries with 150A.
That should shorten their already crap lifespan to just a few dozen cycles.

 

[Russell]

Due to the poor power factor of most chargers it's a good idea to choose an inverter with at least twice the continuous watt rating as the charger.

-R

I also understand that I should expect an initial surge that might be more than the overall wattage needed. But that should only be to operate a piece of equipment for those going camping or something.

In my case of just charging a battery, can I get away with just 20% more than the expected full wattage? That brings up another question. If I only use up half the scooter's 650w of power during my rides, can I technically just buy a 300w inverter and expect that it'll charge the battery completely? or does it totally not work that way? layman's logic. I really have no clue about this stuff. Enlighten me. I'm all ears.

You are confusing battery capacity, measured in Watt-hours, with instantaneous power, measured in Watts. Your battery is 36V (nominal) and 15 Amp-hours giving it a capacity of 540 Wh. This has nothing to do with how big of an inverter you need for your battery charger. To determine that you need to know the power draw of your charger.

Here is an example;

If your battery charger puts out 42VDC at 4A that's 168W DC. Assuming an efficiency of 80% the charger would draw 210W of ac power. Most switching power supply type battery chargers however have a very poor power factor due to the way they draw current. Assuming a power factor of 0.62 (that's what one of my chargers is) the charger would draw 339 kVA. Therefore you would want an inverter that's rated for at least 340W of continuous ac power.


-R

 

[whitep]

I am using a 600w inverter from a power socket in my car . Works a treat.

 

[dogman dan]

Re the wattage. Ok your socket is rated to 300w, so that's a bit of a hard limit to how much power you can pull through it.

But your inverter is not going to pull 600w continuous, unless you are running some thing off it that pulls 600w.

I was assuming a lead acid charger would be 2 amps, so a 48v charger would pull about 100w, then maybe another 50 at most to run the actual inverter.

So that is why I was saying a 300w inverter should do the trick. It's about 2x what the charger and inverter will pull, and not too much for your cigarette lighter.

Get both eventually. Then you can do some charging while driving, but if not enough, fire up the generator at the location that has no plug.

300w inverter is pretty cheap, 40-50 bucks I think.

 

[kmxtornado]

Hmmm, that's good news Dogman. I wish I read this thread before I got back to the office while I was still at Harbor Freight Tools eying the inverters there. I'll give this a go. The worst that can happen is that the cigarette lighter breaks down and I replace a fuse in the car, right? I'm not looking at a possible fire or anything risky like that, right?

If you want to do it cheap, just parallel your scooter batteries and hook them up to the car battery with jumper cables. When charged, put them pack in series.

The only issue with that is sort of a big one. Convenience factor. The scooter I'm referring to is actually a 96lb motorized icechest. The battery pack is a good 24lbs by itself and being able to separate that from the cooler when I transport it in and out of the car for festivals, car shows, street fairs is essential as far as protecting my back when fitting into the trunk on its side. Changing the batteries from parallel to series would mean unscrewing the top off the battery case that houses the 3 batteries each time. Not a big deal as far as effort, but the extra time taken away from the events I'd like to enjoy with my daughter isn't something I'd want to do at this time.

This thing runs on a 12v SLA battery. Is there something wrong? A single 12v battery is charging three 12v batteries? Errrr....wha?

It says it has a 15Ah or 18Ah SLA battery inside.
You'll be able to charge 3 batteries with a third of that into each. Less maybe 20% losses. So 4Ah -5Ah into each of your three batteries.
It can be increased with an external SLA battery.
Doesn't seem like a great option to me.

For cheap and easy I'd try a CCCV Boost DC-DC converter to go from 12V to 42V ish either running off the car (any car stereo or 4x4 installer can fit a fuse and thick power wire for you and a low V battery buzzer), or a spare 50Ah battery.
Or the generator idea.

Can you please elaborate on that? Are you suggesting that I hardwire a DC-DC converter straight off the battery and run it through a firewall into the car? Not sure what a V battery buzzer is. Mind detailing that out?

Due to the poor power factor of most chargers it's a good idea to choose an inverter with at least twice the continuous watt rating as the charger.

-R

I also understand that I should expect an initial surge that might be more than the overall wattage needed. But that should only be to operate a piece of equipment for those going camping or something.

In my case of just charging a battery, can I get away with just 20% more than the expected full wattage? That brings up another question. If I only use up half the scooter's 650w of power during my rides, can I technically just buy a 300w inverter and expect that it'll charge the battery completely? or does it totally not work that way? layman's logic. I really have no clue about this stuff. Enlighten me. I'm all ears.

You are confusing battery capacity, measured in Watt-hours, with instantaneous power, measured in Watts. Your battery is 36V (nominal) and 15 Amp-hours giving it a capacity of 540 Wh. This has nothing to do with how big of an inverter you need for your battery charger. To determine that you need to know the power draw of your charger.

Here is an example;

If your battery charger puts out 42VDC at 4A that's 168W DC. Assuming an efficiency of 80% the charger would draw 210W of ac power. Most switching power supply type battery chargers however have a very poor power factor due to the way they draw current. Assuming a power factor of 0.62 (that's what one of my chargers is) the charger would draw 339 kVA. Therefore you would want an inverter that's rated for at least 340W of continuous ac power.


-R

I see. I see. I knew there was a difference between me charging a battery and all the info I'm reading online about requirements for power an actual piece of equipment. Thanks for reiterating that. I'll take a look at my charger and get the specs on the power draw and try to do that calculation.

I am using a 600w inverter from a power socket in my car . Works a treat.

600w power inverter off the cigarette socket? Doesn't that blow up the socket? I read in a lot of sources that these cigarette lighter plugs range from 150w-400 tops. Most on the low end. Do you have a special type of car? If this is really possible with my car, this sounds like the easiest, simplest, cheapest, quickest way.

 

[kmxtornado]

I got the charger specs. I knew I took a photo of this in case I needed it:

Input: AC 90-245v 50/60Hz
Output: DC 36v Max 1.6A
Equal Changing voltage: 42.6 +/- 0.2v (no clue what this means. My best guess? It charges up to 42.6+0.2v = 42.8v during it's charging cycle?)
Floating Change voltage 41.6 +/- 0.2v (uh, what? Chargers charge slower towards the end of the cycle as I understand it. Doe this mean that during the end of the charging cycle is can charge up to 41.8v?)

Can someone help me make sense of this and draw a conclusion on whether or not I can really use a 300w power inverter out of my cigarette lighter plug? Comparing to what you said Dogman, it's under 2amps and since mine is a 36v charger which is less than the 48v one you put in your example, I'm good to go, right?

Dogman, this is my attempt at calculating everything based on your example. Please help me correct it or verify it is correct. My brain is itching.

36VDC at 1.6A = 57.6W DC
72W of AC power assuming 80% efficiency
116.13kVA (using your same power factor)
So I only need a power inverter that's rated for 120W of continuous AC power?

Big apologies if my math/logic is way off. I'm so dumb when it comes to this stuff. I'm literally just plugging in different numbers based on what I think I understand.

----
If I'm good to go, does anyone have recommendations of brands:
Centech
Cobra
Bestek
Schumacher
Power Bright
Belkin

 

[Russell]

With that charger a 300W inverter should be more than enough. Still as I stated before if your charger is a switching type, and if it's a lightweight brick it almost certainly is, then the current it draws is far from an ideal sine wave which might cause problems with some inverters. I can't make a recommendation other than to buy a good quality model and not the cheapest thing you find on Ebay.

-R

 

[kmxtornado]

Thanks Russell!

 

[Russell]

Oh BTW, You have verified the charger does fully charge the 36V battery when plugged into a household outlet, right?

-R

 

[kmxtornado]

Correct. I did take a multi-meter to it and measured it off the first and third battery and it came up as a bit over 36v when full charged.

 

[kmxtornado]

I've narrowed down my choices to these, both which have amazing ratings of 4.5 stars on Amazon:

1. Bestek BESTEK® 300W (700W peak) Dual 110V AC Outlets Power Inverter Car DC 12V to 110V AC Inverter: $30
Has USB ports too. Leaning towards this one not only b/c of the price, but b/c it has USB ports for charging other stuff and also has some safety shutoff features which is lacking in the one below. The one below also I'm reading is very loud as far as the fan goes.

2. Tripp Lite PV375 Portable Auto Inverter 375W 12V DC to AC 120V 5-15R: $43
This one has slightly higher rating at 375w continuous (600w instantaneous), though maybe that's a bad thing in that it could drain my car battery quicker. 36" cord on this one is longer compared to the 22" cord on the one above. Does not have USB ports

 

[wineboyrider]

Don't forget that many RC chargers can charge SLA my Hyperion 1420 does up to 48v sla all without an INVERTER! :?

 

[kmxtornado]

That's a fairly expensive charger though and it doesn't plug into the cigarette lighter plug, does it? I don't happen to have one already. The RC chargers I have aren't of this variety. Thanks for bringing up the alternative though.

 

[wineboyrider]

Anything over 400 w should not use a cigarette lighter plug in fact,most inverters over that have a wiring harness to connect to the battery terminals. The problem with the inverters is they convert DC power to AC power and then you are charging a DC battery. The losses are high, but with a RC charger you go from DC to DC.

 

[kmxtornado]

Thanks.
I'm excited to order this and hook it up. Plug and play I know. Haha. The fun part will be deciding where to mount it exactly and hiding all the wires to make it a clean installation. I'm thinking either in the glove compartment or under the passenger seat. I usually charge the battery on the floor of the front passenger seat on the go (when I had the 24v battery and the current 150w inverter was working) so either location would probably work just fine. It might be nice to keep it tucked into the glove compartment for when friends and folks want to charge stuff and eventually just leave it in the glove compartment when we arrive (e.g. to a water park or some place where you don't really want to bring your cellphone, tablet, etc and wanted it locked up).

 

[Hillhater]

Agree with Wineboy, ..a 10s RC type charger (>42v) would be a simpler , more efficient solution and could run off the cig lighter socket if required at 2 amps...or it could charge at a much faster rate than your current charger if connected direct to the battery.
Several options available for under $100.

Oh, and I would not put an inverter or charger in the glove box or under the seat whilst in use !