Alternative Backup Option

[aKO]

For personal reasons I have to find an alternative backup method to a generator and considering installing a 24 volt 100ah stand alone lithium battery into our solar system which itself use 18 month old AGM batteries which are performing well . Daytimes we have far more coming in than we need because I have deliberately over panelled so batteries at 100% by mid day and the batteries store more than sufficient to get us through the night , our DOD is no more than 25% , around 80ah however on the odd days we have bad weather the panels manage to keep us going in the day but we have to start a generator for a couple of hours at night to provide around 1kw charge into the batteries plus the 500wh we save on drawing from them in the house while its running , sufficient to get us through the night which is all we normally require .

I know little about Lithium batteries and I keep hearing about things like cell balancing , thermal runaway and BMS so not sure if I'm being nieve thinking you can just connect one of these batteries and use it this way being permanently connected to a small Victron DC to DC charger powered from the bus bar or that it might damage the battery or charger itself if its not designed for continual use , the battery would get little use no more than about 30 times a year mostly during winter months .

My proposal is that when required we could manually switch the power supply to our Multiplus 24/1200 from the battery bank to the Lithium battery rather than the generator as we do now .

I would be grateful for any comments , criticisms or suggestions before I part with any money .

 

[pollenface]

I currently use old random batteries to float charge my good AGMs through the night (keeps them just above 13v).

The process is triggered by the streetlight function of a charge controller and operates via a relay and boost converter. The old random batteries are charged by separate pv arrays/controllers.

I plan to swap the random batteries for lifepo4 in the future (when they die).

A system like that may work for you as you're not really cycling AGMs, but they act as a buffer for your high surge loads.

Food for thought.

 

[aKO]

Thank you Pollenface but im really looking for opinions or advice on the the use of Lithium batteries , if they can be left continuously on charge and suitable for going long periods without use .

Not sure why you dont think i am cycling my AGMs , they are cycled daily , charged during daylight and discharging at night .

 

[Ampster]

looking for opinions or advice on the the use of Lithium batteries , if they can be left continuously on charge and suitable for going long periods without use .

Lithium batteries should not be left continuously at high levels of SOC as would happen if they are continuously trickle charged like Pb batteries. You would need them connected via a different charge controller and something that would effectively separate them from the trickle charging which your existing Pb batteries probably use.
On the other hand, Lithium batteries have very little self discharge which is why trickle charging is not necessary. They are suitable for going long periods without use. They are also more efficient in the sense that they do not need long CV (Absorb) stages like Pb batteries need to stay healthy.

 

[DIYrich]

If you are going to buy a lifep04 battery, I suggest making it your primary battery, and use the AGM for backup. Lifep04 batteries do not have the thermal runway problem of lithium ion battery.

 

[aKO]

Thanks DIYrich , once the AGMs have started to fail i plan to replace them with Lithium lifep04 so its good to know thermal runaway is not a common issue with them however that will be some time away . The AGMs were expensive costing £2000 18 months ago and are four times the capacity i need at the moment to cover us for the occasional backup so would be would be a waste . I estimate i can buy a 24 volt 100ah Lithium battery and a Victron Charger for around £800 which i can continue to use in the same setup once the AGMs have been replaced with new Lithium batteries .

 

[DIYrich]

If your need is 80ah from the agm, then the lifep04 can do that. A battery maintainer can work on the AGM. Use the inverter to recharge the AGM before putting it back on the maintainer. Two 12v agm, or one 24v? Car battery maintainer is easy to find. Cheaper than Victron?

 

[Hedges]

I also have AGM as primary battery, and am interested in lithium which would cycle while keeping AGM at float.

 

[pollenface]

I also have AGM as primary battery, and am interested in lithium which would cycle while keeping AGM at float.

I got the idea from this guy on youtube

 

[aKO]

I also have AGM as primary battery, and am interested in lithium which would cycle while keeping AGM at float.

The problem i can see is that AGM and Lithium use very different charging profiles so would have to have a separate charger of some type . Could use a separate MPPT for the Lithium but then where would it get its solar input , the input i have is one cable from my array and not practical to run another cable to it in parallel and although im over panelled i want to keep all my output available for bad weather days when i need all i can get and more .

 

[pollenface]

The problem i can see is that AGM and Lithium use very different charging profiles so would have to have a separate charger of some type .

You are correct

 

[SamDeleted]

Connecting LiFePo4 and Lead Acid batteries in parallel in RV

I am wanting to change my RV over to lithium batteries but with the expense I have to do it a little bit at a time so I was wondering if I can connect Connecting LiFePo4 and Lead Acid batteries in parallel in RV The same way I connect lead acid deep cycle batteries Currently I have 3 100 amp...

diysolarforum.com

It is possible to parallel LiFePo4 & AGM if you are careful

 

[12VoltInstalls]

considering installing a 24 volt 100ah stand alone lithium battery into our solar system which itself use 18 month old AGM batteries which are performing well

Multiplus 24/1200 from the battery bank to the Lithium battery rather than the generator as we do now

To borrow a term from graphic image compression, AGM- and to a greater degree flooded- lead acid batteries are ‘lossy’ in charging: there’s quite a bit of energy “lost” to heat in charging them. LiFePo, on the other hand, takes virtually all of the energy provided and stores it. Further, lead batteries in general do not ‘like’ extended days of not reaching full charge while LiFePo can stay partly charged and undergo only partial recharge continuously with essentially no negative consequences.

I opened with above for the reason of providing context for a strategy I am going to share.

First: is Multiplus 24/1200 an obsolete model? 1200W, 24V batteries? Why specifically a 24V system for such a small output? I ask because I’m thinking about your goals- 12V LiFePo is an inexpensive solution at 100Ah or 200Ah; I recently purchased a Chins 12V 200Ah for US$599.95. The same output potential as 100Ah 24V. Any inverter <2500W isn’t unreasonable at 12VDC, and you can potentially just parallel two or more 100Ah for increased capacity.
Just to clarify your system requirements.

Are you using the AGM because of a potential for freezing? LiFePo cannot be charged below 32*F or they will be damaged- flooded and AGM batteries do not care. So if freezing is an issue, Lifepo may not be ideal for your plan.

I am very much an advocate for lead-battery use in many circumstances. I ran them successfully for four years.
Recently, I changed over to LiFePo batteries; like you, clouded days can be and were a problem especially winter/late fall.

Because LiFePo batteries do function fine without fully charging daily I decided that overpaneling A LOT would give me some charging even on cloudy days. Because they are fine hovering for weeks at 60% I could survive our local weather pattern.

This is working well. Granted, it’s summer, so more sun hours. However, our weather has been unusually cloudy and rainy this spring and summer so it’s seldom I’m getting much solar charging. My batteries have been holding 13.1-13.4V with a couple 12.9V mornings but I haven’t needed to supplement solar charging at all. Experience under similar conditions is that I’d have had to auxiliary charge my lead acid batteries often. And what finally killed their capacity was 6 weeks of very low state of charge when I had covid and stopped caring for most of that time.
So because LiFePo handles the poor conditions without harm I’ve not needed to supplement charging to them since I think April, maybe March.

In your case I would see if I could see the AGMs since they are still good and buy 300-400Ah of 12V LiFePo batteries to extend your reserve. That way sometime in 3 or 4 days you’ll get sun again to fill them.

You say you are overpaneled now, already. You didn’t specifically mention your Wh of consumption nor how many watts of panels you have. I’m suspecting 800W of panels?
I’d add additional panels and that way instead of

over panelled so batteries at 100% by mid day

…you can recharge earlier on good days but more importantly be sufficient to at least make 75-100% of consumption on bad days.

This would eliminate the need for charging complexity or inefficient transfer of energy from LiFePo to lead(agm).

You’ll still have the generator if you ever need it but your backup will be the extra panels and the redundancy of several ‘smaller’ batteries: if a bms fails or something you won’t be 100% dead in the water.

My opinion. Ymmv

 

[aKO]

To borrow a term from graphic image compression, AGM- and to a greater degree flooded- lead acid batteries are ‘lossy’ in charging: there’s quite a bit of energy “lost” to heat in charging them. LiFePo, on the other hand, takes virtually all of the energy provided and stores it. Further, lead batteries in general do not ‘like’ extended days of not reaching full charge while LiFePo can stay partly charged and undergo only partial recharge continuously with essentially no negative consequences.

I opened with above for the reason of providing context for a strategy I am going to share.

First: is Multiplus 24/1200 an obsolete model? 1200W, 24V batteries? Why specifically a 24V system for such a small output? I ask because I’m thinking about your goals- 12V LiFePo is an inexpensive solution at 100Ah or 200Ah; I recently purchased a Chins 12V 200Ah for US$599.95. The same output potential as 100Ah 24V. Any inverter <2500W isn’t unreasonable at 12VDC, and you can potentially just parallel two or more 100Ah for increased capacity.
Just to clarify your system requirements.

Are you using the AGM because of a potential for freezing? LiFePo cannot be charged below 32*F or they will be damaged- flooded and AGM batteries do not care. So if freezing is an issue, Lifepo may not be ideal for your plan.

I am very much an advocate for lead-battery use in many circumstances. I ran them successfully for four years.
Recently, I changed over to LiFePo batteries; like you, clouded days can be and were a problem especially winter/late fall.

Because LiFePo batteries do function fine without fully charging daily I decided that overpaneling A LOT would give me some charging even on cloudy days. Because they are fine hovering for weeks at 60% I could survive our local weather pattern.

This is working well. Granted, it’s summer, so more sun hours. However, our weather has been unusually cloudy and rainy this spring and summer so it’s seldom I’m getting much solar charging. My batteries have been holding 13.1-13.4V with a couple 12.9V mornings but I haven’t needed to supplement solar charging at all. Experience under similar conditions is that I’d have had to auxiliary charge my lead acid batteries often. And what finally killed their capacity was 6 weeks of very low state of charge when I had covid and stopped caring for most of that time.
So because LiFePo handles the poor conditions without harm I’ve not needed to supplement charging to them since I think April, maybe March.

In your case I would see if I could see the AGMs since they are still good and buy 300-400Ah of 12V LiFePo batteries to extend your reserve. That way sometime in 3 or 4 days you’ll get sun again to fill them.

You say you are overpaneled now, already. You didn’t specifically mention your Wh of consumption nor how many watts of panels you have. I’m suspecting 800W of panels?
I’d add additional panels and that way instead of

…you can recharge earlier on good days but more importantly be sufficient to at least make 75-100% of consumption on bad days.

This would eliminate the need for charging complexity or inefficient transfer of energy from LiFePo to lead(agm).

You’ll still have the generator if you ever need it but your backup will be the extra panels and the redundancy of several ‘smaller’ batteries: if a bms fails or something you won’t be 100% dead in the water.

My opinion. Ymmv

Thank you 12voltinstalls for taking the time to write such a long post so i will do likewise to give as much info on my current system and needs . We already have 2240 watts @ 114 volts of panels supplying 400ah of AGM batteries which we used to replaced the flooded lead acid . Our 24 hour consumption averages around 3kw , can get up to 6kw on rare occasions when im using power tools all day but most comes direct from the solar , even then our AGMs are fully charged and in Float by mid day . I put the system in 20 years ago when we first moved here from Wales , only this year i decided to replaced all the 24 volt Panels , the Charge Controler from PWM to Victron MPPT and Inverter to a smaller Inverter/Charger . I decided on 24 volts originally when our system and consumption was larger to minimise cable losses mostly from the array to the Equipment in our garage . Just my wife and myself now kids have gone so our consumption and loads are low , averages 250 watts load on the inverter at night as we dont have a lot of equipment running , Laptops large LED smart TV Fans and a Couple of Lights and a Fridge cutting in every hour for a few mins , in the daytime can get up to 1100 watts for short periods . The issue we have is bad weather , here in Spain its normally just the odd day in the winter and while the over panelling manages to handle our load in the day and we have no problem limiting our usage what we use we want to be able to do is maintain our normal consumption through the evening and night , you mention reserves of 3 or 4 days , for us its normally just a day the following day were normally back to good weather , when people ask us how Spain compares to Wales we just say that in Spain when it rains we know its not long until weathers back to normal , in Wales when theres a dry sunny day we say the same , not long and weather wil be back to normal . Our total consumption between 6pm and 7am for the last year or so has maxed at 93ah , approx. 2300 Wh but normally averages around 60ah ( 1500Wh , )DOD has never been greater than 20% but of course thats always starting the evening with full Batteries .A 100ah Lithium battery i am thinking would provide that . At the moment we would start a Honda EU22i inverter generator and run it for a couple of hours around early evening when were watching TV , the Multiplus Charger/Inverter puts in 40-50ah during that time , this is what i would like to avoid by having an alternative option , the possibility of flicking a switch when the AGM get low after a poor day is worth the money and i dont see it as being complicated , if its practical to install a Victron DC to DC battery charger and leave it connected to the Lithium batteries to keep them full and ready for the odd occasion then its something i am prepared to do , knowing very little about lithium batteries in this situation i just need to confirm everything before ordering everything . I messages a couple of Lithium battery suppliers on Amazon Spain and they say better to charge with their charger , that come included with their batteries , they state that they can be left connected continuously but of course the chargers are always 230 volt AC , i prefer to stick with DC charging . I am thinking that when our AGMs eventualy need replacing i wil go for Lithium and keen the ones we buy now as they wil be used , as backups , no need to change that aspect of the system , they will stil serve the same purpose although they might not get much use .

 

[12VoltInstalls]

Ok. Whatever route seems appropriate for you.

I’m totally amazed your AGMs are delivering what you described! I had no idea you had that many watts of solar based on my experience with flooded batteries in boats and jeeps- and watching others with AGM not get that much over 50% the life of my cheap flooded versus their AGMs that cost 2-4 times as much.

2kWh of LiFePo seems like a good backup, I’m running 4.8kWh at 12V with none of the lead battery concerns which was my goal. Plus, I don’t cycle them very deep at all.

If you do what you describe, let them rest at 80% charged (20% discharged) and fuse them properly.

 

[Hedges]

The problem i can see is that AGM and Lithium use very different charging profiles so would have to have a separate charger of some type . Could use a separate MPPT for the Lithium but then where would it get its solar input , the input i have is one cable from my array and not practical to run another cable to it in parallel and although im over panelled i want to keep all my output available for bad weather days when i need all i can get and more .

I don't mean AGM and LiFePO4 in parallel. Separately charged and discharged.

I have AC coupled GT PV and battery inverters with AGM. Grid backup.
When off grid, GT PV responds to frequency-watts, ramping down production from 61 Hz to 62 Hz, hovers in between to match loads and charging.

What I'd like to do is AC couple a bidirectional inverter (e.g. Sunny Boy Storage) with LiFePO4.
Configured to draw from battery and produce AC, frequency-watts ramp-down from 59 Hz to 60 Hz, then ramping up battery charging from 60 Hz to 61 Hz. If less power is needed, first power draw is curtailed from LiFePO4, then charging of LifePO4 is ramped up. If still too much power, then PV is curtailed.

I don't know if SBS can ramp charging in response to frequency. I does ramp output, according to Rule 21.

Same idea might be done DC coupled, with DC/DC drawing from LiFePO4 to charge AGM and keep it at float. When there is surplus PV, charge LiFePO4 as a dump load.

The goal is to keep AGM at float, discharging or charging LiFePO4. AGM just supplies surge power (and cycles only if LiFePO4 is drained, or power draw exceeds its inverter wattage.)

 

[aKO]

Ok. Whatever route seems appropriate for you.

I’m totally amazed your AGMs are delivering what you described! I had no idea you had that many watts of solar based on my experience with flooded batteries in boats and jeeps- and watching others with AGM not get that much over 50% the life of my cheap flooded versus their AGMs that cost 2-4 times as much.

2kWh of LiFePo seems like a good backup, I’m running 4.8kWh at 12V with none of the lead battery concerns which was my goal. Plus, I don’t cycle them very deep at all.

If you do what you describe, let them rest at 80% charged (20% discharged) and fuse them properly.

I hope the information i have passed on is correct , i have taken it from Victron VRM although voltages i used to worry about because when the batteries are on load its misleading and they appear lower than they are . I installed a Victron Shunt and those are the figures im seeing now , amazed that the voltage can go down from 25.6 full charged when charging ends to 24.6 during the evening then when we go to bed turning off the TV , lights , Laptops etc and within a min the voltage is around 25 volts so im very happy that i decided to buy the shunt , made we worry a lot less that i was taking to much out of them at night . Consumption on VRM was in Watts but agrees with Shunt which is in AHs

 

[pollenface]

Are you using the AGM because of a potential for freezing? LiFePo cannot be charged below 32*F or they will be damaged- flooded and AGM batteries do not care. So if freezing is an issue, Lifepo may not be ideal for your plan.

I noticed during very cold days that the temperature compensation pushes the AGM voltage up to near 60v (48v system) which is when the inverter will shut down to protect itself. That could be a worry.

Hypothetically i could drop the absorption voltage a tad to compensate.

 

[aKO]

Batteries and DC-DC charger ordered , now i jusy need to confirm how to wire everything . Looking for a change over switch ( centre of diagram )so that i can turn Off the feed to the Inverter from the BusBar and make a connection to it from the Lithium Battery , what type of switches would normally be used , i can find isolation switches but they are simple on/off , the ones i can find are AC and i dont think they would handle the amps , i need to break one circuit and make another . Is it safe to use the same BusBar for both the main MPPT to AGM circuit and the DC-DC charger with Lithium battery or would it create a conflict with two chargers producing power at the same time .