For RV: A couple of questions on my cheap DIY build

[Timtumm]

In case it matters, I have already bought 4 3.2V-280Ah batteries. They look good.
The very small RV build (on a short, 118"-wb Promaster) should need very little power (only for lights, a fridge, a fan, and

1. Every body is using the Victron 121230 DC-DC convertor/isolator. But is is so expensive (over $200). I have been buying cheaper stuff for everything else, and hate to pay almost as much for this usage as I did for the batteries themselves. Is there a cheaper alternative?

2. I also cheaped out, and bought a dinky BMS, which looks way too small now that it has arrived (2"x2.5"). I guess size is not everything, but there is no documentation. The marking on it are:
4S 120A 3.2V BMS Connections: Same Port (whatever that means)
Charge Current Max 60A Working Current: Max 120A
Heltec-BMS.com SKU 0914 Heltec BMS
Can I use this? I ask because it only has 4 power wires for the batteries, and I also expected a 5th for the ground.

2A. If I can't use this BMS, any recommendations for a cheap BMS?


3. Any recommendations on an inverter? I am assuming 1000-2000 watts for charging laptops. I also assume that because of this, it needs to be full sine wave.

 

[hwse]

Others might disagree but if the laptop charger is just an AC / DC transformer, it probably does not need to be full sinewave because it will just be rectified back to DC anyway. If it is one of the smart bricks that have electronic charge controllers, that might be a different thing.

 

[grizzzman]

There is this dc to dc https://www.amazon.com/Renogy-Battery-Batteries-Multi-stage-Charging/dp/B07Q5VYPCF/ref=sr_1_22_sspa?crid=EHW6I1O6P6O1&keywords=renogy&qid=1645308630&sprefix=renogy,aps,185&sr=8-22-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyWFYyS1JIS0NDQ1ZGJmVuY3J5cHRlZElkPUEwOTQ0ODUwMkZXQ1JEU1ZDNzJJMiZlbmNyeXB0ZWRBZElkPUEwOTc1MDExMU9RTjZMME5BVFY3NiZ3aWRnZXROYW1lPXNwX210ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=
If you install a 2000 to 1000 inverter then your BMS is not big enough.

 

[jdfrye]

I would say that you are making your own battery why would you not want to have a Bluetooth BMS? I love being able to control the charging and the discharging on my new battery I built. I'm in love with the functions it gives me.
I know about the DC2DC charger..it hurt me to spend the money on the Victron Orion 121230 but it is solid!
Again the BT on the JBD BMS I bought is the highlight of my build. You can find the BMS APP on overkill site and also buy 120A from them. I waited 3 weeks for my 150A from this link

67.59US $ |JBD Smart Bms 4S 150A With Free Bluetooth Temp Sensors 60A 80A 100A 120A Lifepo4 BMS Board Supports Connect In Series DIY E Tool| | - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

 

[rmaddy]

Any recommendations on an inverter? I am assuming 1000-2000 watts for charging laptops.

Do not use an inverter for charging laptops. Your laptop actually wants DC power. Your battery is DC. Why go from DC to AC (via the inverter) and then from AC back to DC (via the laptop's power brick)? Wire up a standard 12V car power plug or two. Then you can plug in standard USB chargers for your phone and laptop. Far more efficient and no need for an inverter.

 

[Timtumm]

First, thanks for all of you who replied.

@hwse It is something I heard; I could be wrong. I also have a dinky half-sine wave inverter and I have tried it, and it would not charge my laptop (but it could also be too low-power).

@grizzzman My batteries at LiFePO4. Your DC-DC says it is good for "Flooded, Gel, AGM, and Lithium". Am I still good? Am I Lithium?

@jdfrye I am trying to make a budget build. I know I won't be using the BT app, because there will be a battery monitor I can look at.

@rmaddy My laptop is a dell. It can't use USB. It needs like 14v DC, and a proprietary connector, my wife's laptop needs somethings else.

 

[grizzzman]

Yes I run the 40 Amp version.

 

[chrisski]

If you’re going to trust that BMS to monitor your batteries, that is your choice. I have two 24 volt 25 ah battery packs I keep charged with a 30 amp discharge 15 amp charge BMS. Make sure you know your BMS limits, and accept the risk which means cells swelling or even worst.

My BMS has terrible settings which are non-adjustable. Low voltage cutoff is below 2.5 per cell and high volts is above 3.65 VDC which is the published max / minimum on the cell spec sheets. Also the BMS has a low temp cutoff below the 0C the cell spec sheet has.

So starting off, I can’t trust my BMS for nearly every spec except over amps. I have no real protection against a runaway cell, under voltage, low temp cutoff until its too late. I top balanced my cells, try I not torun the battery dead (but I did once), and make sure its above freezing when I use it. I have a battery monitor, but I can use this pack for 12 hours and only look at it a couple of times.

When I use this battery pack, its set up outside where if it catches fire, the house won’t burn down. If you’re comfortable with that BMS and accepting risk, then by all means go for it. I would not do that for a van build.

My bigger battery packs, two 8S 280 ah 24 volt batteries, have a blue tooth BMS I can monitor and adjust settings.
===================
The batteries I mentioned are charged off solar only, and I have no way to charge them without solar, so I don’t have a DC to DC converter for them.

Some RVs come with a 13.6 VDC constant voltage converter and this may be good for your lithium battery and may not come out. Whenever I monitored my 13.6 constant voltage converter, the volts never went over it and it never put out more than 10 amps, so if you can turn this thing on and off easy enough, it may work. I used this charger I mention to charge my lead acid batteries in bad weather, but Ihad to take it out for the 24 volt build.

 

[camelCase]

For charging your laptop, you can get something like this:

https://www.amazon.com/Universal-Charger-ThinkPad-Notebook-Computers/dp/B095SF992Q/ref

Probably more efficient that using an inverter.

 

[chrisski]

THere’s so much you can use an inverter. I was told to plan for 250 watts, or around 21 amps on a 12 volt system, and when I charged a laptop it was around 140 watts or 10 amps. I don’t know if my 300 watt inverter would work, but I know my 2000 watt inverter does. Most inverters I looked at lose 15% on the DC to AC side and I’m sure there’s a similar amount going back to the DC side.

If your laptop us your biggest load, a 1000 watt inverter and a 280 ah battery pack may be a good match. I could charge that laptop for 10 hours and use 10 ah on AC, and perhaps 60 ah if it was DC only. May or May not tip the scale. 10 hours a day on a laptop is a lot. Apple tells me I spend about 3 hours a day on my devices.

 

[rmaddy]

THere’s so much you can use an inverter. I was told to plan for 250 watts, or around 21 amps on a 12 volt system, and when I charged a laptop it was around 140 watts or 10 amps. I don’t know if my 300 watt inverter would work, but I know my 2000 watt inverter does. Most inverters I looked at lose 15% on the DC to AC side and I’m sure there’s a similar amount going back to the DC side.

If your laptop us your biggest load, a 1000 watt inverter and a 280 ah battery pack may be a good match. I could charge that laptop for 10 hours and use 10 ah on AC, and perhaps 60 ah if it was DC only. May or May not tip the scale. 10 hours a day on a laptop is a lot. Apple tells me I spend about 3 hours a day on my devices.

I'm curious why you think a 300W inverter would have a problem with a 140W laptop charger.

Otherwise this post shows why you should avoid using an inverter when direct DC charging is so much more efficient.

10Ah on 120V AC means 10Ah x 120V = 1200Wh.
60Ah on 12V DC means 60Ah x 12V = 720Wh. That's nearly half the power.

 

[jdfrye]

First, thanks for all of you who replied.
@jdfrye I am trying to make a budget build. I know I won't be using the BT app, because there will be a battery monitor I can look at.

Not sure I would budget on the BMS.
You would want a quality BMS for you investment.. NO?
Am I being a jerk?

 

[chrisski]

I'm curious why you think a 300W inverter would have a problem with a 140W laptop charger.

I don’t think it would Have any issues. I just can’t speak from experience because I have not used my 300 watt inverter to charge A laptop; only my 2000 watt inverter.

Otherwise this post shows why you should avoid using an inverter when direct DC charging is so much more efficient.

10Ah on 120V AC means 10Ah x 120V = 1200Wh.
60Ah on 12V DC means 60Ah x 12V = 720Wh. That's nearly half the power.

Undoubtedly the DC route is more efficient. I think every 12 volt build should have a 500 watt or 1000 watt inverter, especially if it has 280 ah of battery pack. THere’s conveniences to be had in life. It’s OK to have an inverter. Nothing wrong with watching some TV, etc. So this is not the most rational choice, but AC power lets you enjoy more things in life.

How much this laptop is actually used will decide whether a reasonable amount of energy is saved or not. If this laptop is only on one or two hours, its not a whole lot Of difference between AC and DC energy wise with a 280 ah battery. If this laptop is going to be left on all night to game, then I whole heartedly agree DC is the best route to go.

 

[Timtumm]

I have done more reading. I clearly need a larger inverter than I thought, since I would like to run a 700 Watt Microwave (I think that is as small as they come). I can time it so it is the only (AC) thing running at the time. I have also read that for a Microwave, I need Pure Sine Wave, or I would get a buzzing sound.

I am OK throwing my $20 BMS away and buying a better one, if that is the wisdom. Any recommendations?

 

[chrisski]

Need to figure out what inverter will power that 700 watt microwave. You’ll need more than a 700 watt inverter. THe 700 watts is cooking power and it will pull a bit more from the inverter and even more from the batteries

For discussion, if that is a 1200 watt inverter, you’ll need a BMS capable of pulling over 110 amps at 12 volts continuous and wiring to cover that.

I have a bigger 1100 watt cooking microwave I ran off a 2000 watt inverter that pulled 135 amps from thebattery when cooking.

 

[acdoctor]

First things first through the Dell away. I had 1 Dell, terrible. No I have been using laptops and printers in a work truck around 20 years. The old ones would run their battery dead even when sleeping. I used a 500 watt square wave inverter on them. At some point 12 volt charge cords became available. Then solid state hard drive. They can sleep for days on their battery. Also battery powered printer. Now I don’t carry any cords or inverter. Charge laptop once a day. Printer once a week.
Cheap electrical work usually means fire or failure. Running a microwave or A/ C is the hardest things to do in a RV just below that is a hair dryer. Electricity is a balanced equation. Higher voltage lower amperage. If you build with cheap components and try to pull higher amperage the voltage will drop which will cause amperage to increase. Amperage causes heat through resistance in budget components. The higher the resistance the lower voltage in vicious circle until something trips, melts or burns. Most US house’s don’t pull 100 amps. If they do it is for a short period. 100 amps on Any voltage has to be done right.

 

[12VoltInstalls]

You’ll need more than a 700 watt inverter.

I have an old hand-me-down microwave in storage. I don’t use it. It is broken and will only run with “1 minute” shortcut button. My 1200W Giandel started it once for five seconds; I should have tested longer and read loads but didn’t. I just wondered if it would start.

If you want to use a 1500W inverter intermittently, I would double up on your battery myself. (400W of panels would be ‘small’ but help keep it going), and as far as DC2DC charger: those are cheap over time as it should last years and a proper charging profile gives you the best battery performance.

On the cheap end, but not poor quality, I have a Giandel 300W pure sine inverter that draws 0.26A no load. It charges cordless tool batteries, laptop, and runs my little soldering iron when not at home. Wonderful little tool for like $50

 

[jdfrye]

I am OK throwing my $20 BMS away and buying a better one, if that is the wisdom. Any recommendations?

Current connected has the JBD 150a in stock looks like, otherwise you can get it in the other link on I posted from China and hope it gets to you in a timely manner

JBD BMS 24V/100A (8s) | Current Connected

The JBD BMS is designed to protect your LiFePO4 batteries and cells, ensuring maximum lifespan and performance of your energy system!

www.currentconnected.com

Overkill has the 120a model only

 

[jdfrye]

I have done more reading. I clearly need a larger inverter than I thought, since I would like to run a 700 Watt Microwave (I think that is as small as they come). I can time it so it is the only (AC) thing running at the time. I have also read that for a Microwave, I need Pure Sine Wave, or I would get a buzzing sound.
?

Will who is an Admin on these forums makes videos. Here's one that will help you on your decision for a inverter

 

[rmaddy]

I clearly need a larger inverter than I thought, since I would like to run a 700 Watt Microwave (I think that is as small as they come). I can time it so it is the only (AC) thing running at the time. I have also read that for a Microwave, I need Pure Sine Wave, or I would get a buzzing sound.

A 1500W inverter should be enough for a 700W microwave oven. A 2000W inverter should handle a 900W-1100W microwave.

Even with the inverter your goal should be to run as much on pure DC as possible as I've already pointed out. Your batteries will last longer between charges.

Inverters drain power even if nothing is plugged in or being powered by it. So you will want to only turn on the inverter when you actually need AC power.