Charging Lithium Ion battery from Lead acid

[Robmc]

Hi
First time poster, so tell me if I am on the wrong part of the board.

I am working on a small project. I want to charge a mobile lithium ion battery which is 36V10ah, with a lead acid battery which will act as the battery bank.
I know I will need my lithum Ion battery charger and an inverter to do this. But my question is the size of lead acid battery I would need, I am going to use a SLA or Gel cell.

I want a lead acid battery that will have at least a life of 5 years. I also want the size of battery that can fully charge the lithium ion in one sitting without being damaged by the depletion, so a slightly larger capacity of lead acid than the lithium ion.

Can you tell me what size lead acid i would need or how I should work this out.
thanks
Rob

 

[DThames]

Rob, I think most people will tell you that if you want long life then you should be using LiFePo4 batteries and not lead acid.

You said you want to charge your 36v battery in one session but you didn't say how long your main bank would need to recover or how it will be charged. If you are planning to solar recharge the main bank, then maybe 5 to 10 times the size of your 36v would be a good starting point. 48v 100Ahr would be standard LiFeP4 size. This might be overkill but would deliver several charge cycles without being refreshed.

 

[krby]

1. You can save some wasted power by not using an inverter, find a DC-DC lithium charger and connect it directly to the lead SLA batteries

2. As for how SLA capacity you need, you can calculate it this way:
- Lithium is 36V x 10Ah = 360 watt-hours (Wh)
- SLA should not be discharged more than 50%, so you need at least 720Wh of SLA capacity
- If you SLA bank is 12V then 720Wh / 12V = 60Ah, if 24V SLA then 30Ah, if 48V SLA then 15Ah.
- If it were me, I would round up a bit to give yourself more headroom. So maybe instead of looking for 720Wh of SLA capacity, look for 800Wh or 1000Wh

 

[Robmc]

Hi Gents

Ok food for thought. Thank you both for helping out. I am working to a budget so I cant go rolls royce but at the same time I dont want bargain basement. I am happy to pay a little bit more for system that is reliable, I cant have breakdowns as the systems will be remote.

So the first answer. I had presumed lead acid would be a reliable and cost effective option for a power bank and more cost effective than a LifePo4 (lithium ion) battery. But maybe that the first question I need to ask.

So can I get your thoughts on that. It would obviously men a solar lithium ion controller instead of a standard solar controller. Weighing up the costs and reliability.

Again thanks for your patience and Merry Christmas

Cheers
Rob

 

[DThames]

I think by 500 discharge cycles (you should look it up) the lead acid will show signs of reduced capacity. You don't want to draw down lead acid but about to 50% discharge or you will shorten their life, so you end up (often) with a larger size which costs more.

You didn't share what your goals and constraint are but without more information I can't help but wonder if 3 or 4 of your 36v packs in a charging rotation would not be better than a large bank being recharged and then that bank recharging a smaller battery . You will lose some in every transfer. You will loose power to charge your bank, then loose power to charge your 36v battery. If you had a 36v battery in use and 2 or 3 more charging, like you would need a fresh one each day and the others would be 1 day old on the charger, 2 days old on the charger, etc and just rotate those 3 or 4 without a large bank charging and dumping, would that be worth looking at?

 

[SCClockDr]

As you are starting from scratch I'd suggest you start with educating yourself about lead acid & LiFePo4 battery properties. Watch some of @Will Prowse videos.
It would assist us to understand the frequency this battery to battery charge evolution & the limitations leading you to consider this type of solution.

 

[Robmc]

Hi Gents
Sorry I have been offline over the holidays. I will share more of the project with you. I was just trying to do bite sized chunks, otherwise there are sometimes too many variables in the whole project.

The 36V10ah battery is in an electric bike. I am trying to set up a community ebike system in our suburb to cut down on car use and try to educate people that there are other forms of transport rather than hopping in a car, especially for very short trips, less than 5km.

So the 36V10ah lithium ion battery is going to be depleted approx 20% each day in the bike before it is connected back to to powerbank battery to recharge the depleted 20%. From reading your messages above I know understand where there may be benefits to lead acid for the powerbank but there are also downsides of usable power vs damaging the battery. So thanks for clearing that up, I now know for better life of the powerbank battery I should go for lithium ion.

So in theory I could have a matching battery in the powerbank as it would take 5 days without the powerbank battery receiving any charge before it is fully depleted. BUT I would prefer that battery to have some additional capacity and less chance of damage, So I would go for a 36v20ah battery in the powerbank.

My next question. Normally I would have a DC powered charger to charge the Ebike battery, but I will need an AC powered charger to charge the Ebike battery. Any recommendations, it would be very similar to a solar controller I presume.

Thanks
Rob

 

[krby]

To charge Ebike from power bank you should use a DC-DC charger. If you avoid the inverter here, you'll lose less power to conversion efficiency.

How are you planning to recharge the the power bank? Solar? AC? This will determine what kind of charger you need for the powerbank

 

[Robmc]

I am planning on solar, and that will be the next part of the question.

Average full sun for my location is 4 hours per day, this takes into account winter and summer. My next question is size of solar panel to charge the battery, i was aiming for a 100watt panel @ 18V, giving 5.5 amps per hour. Would I be correct in thinking this would charge a 36V20Ah battery in 4 to 5 hours of full sun.

 

[DThames]

You have to put in more than you can take out because there are losses both ways. There are losses in going from the panel voltage to the battery charger, like if you charge 36v with an 18v panel, you have to do a DC to DC conversion built into your charger. The 100w panel will not get 100w very often, even in good sun. So you error on the side of too big in all estimates. Note someone else might have a better way to do this estimate, that includes the normal losses.

You should convert what the panel can deliver per day to watt hours. So say 400 watt hours, to stay with your numbers of 100w and 4 hours per day. 400 watt hours would be your panel output estimate. Volts times Amps is Watts. Pulling 20 amps at 36 volts will be 720 watts. If you do that for one hour that is 720 watt hours. So with two good days of sun you might come close to charging the battery if it was fully discharged.

(questions are rhetorical)
In a real world situation things will be different. For example will you fully discharge the battery every day? Will the depth of discharge be a lot more in the summer because of pleasure use and will the longer sunlight hours of the summer make up for that. Is it cloudy all day for days in a row when it is cloudy?

You should consider a much large panel or more panels. I don't know where you live but if you have the room for larger panels, look at used commercial panels. A new 100w panel would cost me about $80 (US). I purchase some used 295watt for $60 each. Shipping for one large panel is too expensive but if you could find one locally that might be a good thing to look for.

If you have to fully charge every day, I think 300w range would be a good starting place, considering all of the losses.

 

[pollenface]

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