Questions regarding battery type and what voltage moving forward

**venquessa** · 03-30-2023, 10:19 AM

You won't be able to just connect the two batteries anyway. You will need a DCDC converter to limit the current between the two... or a battery charger.

I have a very similar system, but used for household battery charging and 12V loads. I have upgrade to 105Ah LFP cells from AliExpress and I am not looking back. In fact I ordered another 4 to go up to 8S and 24V.

There are many options out there for 24VDC (or 12VDC) chargers which themselves run off 12V or 24V DC.

That limits you to charging one way at a time between the two. If it's infrequent you will change over, then it's probably the cheapest option.

Paralleling LFP and Leadacid won't work. The lead acid will just slowly drain the LFP.

By directional DCDC converters could work, but unless you are at least beginner level skilled in electronics its probably not worth attempting.

With them being all lead acid, you still have to be careful putting them in parallel. They need to be in approximately the same state of charge as each other or you can get excess current flow between them. Once they are brought into sync they should stay in sync though.

If you went lithium for both the same would apply, although surprisingly lithium LFP cells are not affected as much by this.

People sometimes use a battery charger to bring the low up towards the high then connect, some people calculate a resistor value and put it between them for a few hours first. YMMV

**venquessa** · 03-30-2023, 10:22 AM

Remember that a 4S LFP pack with a durable BMS system will happily get treated like lead acid in 99% of cases. Niether load or charger should care it's LFP.

**georgia088** · 03-30-2023, 10:57 AM

This is awesome info! I am an electronic hobbyist. Although far from an expert, I do love dabling in it. I am not sure I completely understand what you mean by directional DCDC converters, but this is kind of what I am thinking after doing a little more research. Please correct anything I am doing wrong.

Get another 100 watt panel (3 100 watts total) Wire them in series. The panels have a Vmp of around 20.3V. This should put me at about 15A total going in to the charge controller(s).

I am going to have the panels feed the 20-30A charge controller. Out of the charge controller, I am going to go to my 24v system that is on the boat for trolling and have a "plug in" connected.

From the 24V system (not sure I can do it out of the charge controller) I am going to connect a 24 to 12 V DC step down converter. Out of the Step down converter, I am going to run in to a second controller. The second controller will be responsible for keeping the LIFEPO4 12v battery charged which will be mounted permanently to the dock.

If I am thinking correctly (very possibly not) This should isolate the 12v LIFEP04 from the 24v lead acid. I may have to add diodes to the setup to keep the voltage only going in one direction.

Does this sound feasible? I will try and draw a rough sketch of what I am thinking.

Also, I am looking at ordering the cells from aliexpress as well. Could you provide a link or a model number of the cells you got and are happy with?

I really appreciate the help!

Thanks.

**georgia088** · 03-30-2023, 11:08 AM

Originally posted by venquessa

Remember that a 4S LFP pack with a durable BMS system will happily get treated like lead acid in 99% of cases. Niether load or charger should care it's LFP.

here is a rough drawing of what I am thinking. I’m not sure if it would be better to wire the panels in series or parallel. I already have a couple of mppt controllers, but I’m not sure what there voltage rating is for. If I wire in series, the voltage may be too high to use my charge controllers.

Attached Files

solar.jpeg (509.5 KB, 93 views)

**venquessa** · 03-30-2023, 11:48 AM

Panel series voltage is a limitation of the MPPT controller. On the back of the panel you will find it's "open circuit voltage" or VoC.

12V panels tend to have an actual power point between 15 and 17V and an open circuit voltage of 20V or so.

The MPPT controller will have maximum for the "Panel series voltage", normally this is 100V or 150V or higher. I haven't seen a smaller one.

Additionally it will have a charge limit (20A I think you said). It is "likely" do comfirm, your MPPT will handle 100V and 20A, however it may not charge the batteries at that rate, it may de-rate to it's maximum wattage. Mine is 100V, 40A, but it cannot do 4000W. It can support up to 1500W of panels, but only 1000W battery charge.

As to your diagram.

I would swap the LFP and the 24V Lead acid. Change the DC-DC converter + charge controller to just a "12V lead acid charger" unless that is what you meant by DCDC converter and "charge controller".

The LFPs are better suited to constant cyclic use, it will tire the lead acid quickly. Even deep cycles are only quoting 800-1000 cycles, which is 2-3 years. LFP quote 3000 cycles or an 8 year life.

Panel string voltage versus battery voltage is less important, but cannot be ignored. A large difference can make it trickier for your MPPT and less efficient.

**venquessa** · 03-30-2023, 11:51 AM

Also consider that a 100Ah lead acid will only ideally (for it) only give you about 50-60Ah before you start to stress it. An LFP pack won't care if you flatten it, as long as you don't "under volt" it. It should also provide a far, far more stable voltage at high current output.

**venquessa** · 03-30-2023, 11:55 AM

Don't know if this helps, but I'm experimenting with 24/7 loads on my 12V system. It has been running a 20W network switch for a week. I added a 10W Wifi Router to it and the weather has turned murky for 2 days.

So, I am going to attach an electronics bench supply set to 13V to the battery, limited to 5A.

Should the load being the battery below 13V, the power will begin to be supplied by the bench supply running off mains. It's not efficient. The PSU comsumes a few watts even idle.

These kinds of boost/buck power supplies are around £50 on Aliexpress for a nice one with a screen, case etc. They tend to run on "whatever power". A wide range. Mine takes anything from 5V to 50V and puts out the same range regardless of the input. 5V in, 50V out, or vice versa, no problem. If buying, check it supports battery charging. There is a single component difference (a diode) on the input that will vapourise if present, in the wrong config when you connect a battery.

**georgia088** · 03-30-2023, 12:01 PM

Originally posted by venquessa

Panel series voltage is a limitation of the MPPT controller. On the back of the panel you will find it's "open circuit voltage" or VoC.

As to your diagram.

I would swap the LFP and the 24V Lead acid. Change the DC-DC converter + charge controller to just a "12V lead acid charger" unless that is what you meant by DCDC converter and "charge controller".

Are you saying to have the LIFEO4 12v battery "charge" the 24v (2 deep cycle 12v batteries). So, basically the solar panels go to the 12v battery first and then I step up the voltage to charge the 24 battery?

Also, do you have a model number or link of the LIFEPO4 cells you got?

Thanks!

**georgia088** · 03-30-2023, 12:05 PM

Originally posted by venquessa

On the back of the panel you will find it's "open circuit voltage" or VoC.

Also, One of my panels has an Open Voltage of 24.3V.

**venquessa** · 03-30-2023, 12:25 PM

https://www.aliexpress.com/item/1005002848887065.html

You should do a bit of research, some people are more picky than I. I notice that one of my cells has either a slightly different capacity or a slightly different internal resistence. That's not a problem as long as you also fit a BMS to them, which you should do anyway as it will save you backside in short circuits etc.

I believe, if you have the money, the dogs proverbial are the EVE-Automotive Grade 310Ah cells. But expect to pay about £250-300 per cell! However that's over 1kWh in a single cell!

Andy at "Off grid garage" on YouTube is a better source of info on where to buy the best cells.

Batteries and ordering.
I think either way will work. It's up to you. The limitation really is that one set of batteries becomes "load only". You can charge them, but not run the system off them. Again that might be fine.

It really depends on the details of how much sun you get, how big the 24V lead acid is, what is your budget for lithium. Etc.

**georgia088** · 03-30-2023, 12:35 PM

Originally posted by venquessa

https://www.aliexpress.com/item/1005002848887065.html

Batteries and ordering.
I think either way will work. It's up to you. The limitation really is that one set of batteries becomes "load only". You can charge them, but not run the system off them. Again that might be fine.

Are you saying load only because I will only be able to physically wire up my system to either the 24v or the 12v battery? I was trying to come up with a way to be able to switch it. So, say the boat is stored under the dock most of the time and I want to be able to use the 24v system (or 12v) until the battery goes below a certain threshold and then "switch" to the other? Honestly, I was thinking of possibly programing a relay to do the switching, but I haven't quite wrapped my head around that yet.

I'm not that picky either. I like the batteries in your link. I am not looking to spend a fortune on this project. Thanks again!

**venquessa** · 03-30-2023, 12:54 PM

Yea. I was trying to think about that too for you.

Switching yes. There would be a rather simple way to do that as well. A standard A/B/OFF boat power selector. NOT an A/B/Both switch!

Like this (3 pos version):

https://www.aliexpress.com/item/1005002565045590.html

I believe this is used on small boats to switch between dual backup starter batteries or on larger boats to switch between house a batteries and starter battery..

Just common all the negatives. If you want protection, you need to add it to both batteries.

The MPPT controller should fix itself up instantly and not be a concern. You loads however would switch between 12V and 24V unless you also regulate the output to one or other.

I was thinking about bi directional chargers set to auto engage, one or other. Which ever battery is high, charges the other. It's, I think just too complex, would need to DIY and if you get it wrong and run both together you just flatten both batteries slow running current around in a loop all day.

**venquessa** · 03-30-2023, 12:55 PM

Relays. Possibly. However you MUST research them carefully. The batteries are more than capable of 100A+ under load. You would ideally want a dual pole "break before make" relay. At 100A that's more likely going to be a "contactor" which itself is adding complexity.

**venquessa** · 03-30-2023, 12:59 PM

Sorry. One tiny corner case. Check your MPPT is not "common positive" before common-ing your negatives. I do not know the consequences!

Questions regarding battery type and what voltage moving forward

Questions regarding battery type and what voltage moving forward

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