Solar charged battery question - very sporadic use... - Page 1
Forum section: electronics / general

author: jmh
author_rank: Contributor
timestamp: January 27, 2025, 02:49:34 pm
content: Hi allI am looking to sort out batteries located in the middle of nowhere that drive a railway point machine. No mains available. The batteries live outdoors in a metal cabinet. At the moment we have 3x 12V 110Ah leisure type flooded batteries charged by a Renogy 10A MPPT solar charger. I can’t remember the spec of the solar panel but it has proved sufficient for some time.The issue we have is that if one or more batteries drop sufficiently low for whatever reason the charger gives up, so they drop all the way and we need to replace them (and hopefully charge the duds back at base) which is a pain as it’s 2.5km from anywhere and a long walk! Last week one was 12V and the other two only 9V so not good. We think probably the panel got covered in snow so there was no charge for ages.The point machine takes (I think) 7A at 30V (we give it 36) for about 10 seconds per throw. The points only need to be thrown a maximum of 6 times per day, but not every day with many months in between depending on what events are being run. They might not be operated at all for 6 months other than for maintenance.I am wondering if replacing the three with a single 36V LiFePO type would be sensible or stupid. The Renogy charger has a setting for lithium. But my concerns include the fact it would be sitting at 100% for maybe 6 months, and how to ensure it cannot be drained to 0.Opinions would be most welcome! I am waiting for a reply from Renogy but I bet they just say we’re doing things it is not designed for…Thanks!

author: Marco
author_rank: Super Contributor
timestamp: January 27, 2025, 03:28:35 pm
content: Can't you double the panels and mount them vertically?

author: tunk
author_rank: Super Contributor
timestamp: January 27, 2025, 03:36:31 pm
content: Place the panel more vertically to lessen problems with snow?Or maybe use a 12V system plus a step-up converter?

author: jmh
author_rank: Contributor
timestamp: January 27, 2025, 03:59:29 pm
content: Quote from: tunk on January 27, 2025, 03:36:31 pmNow there's a thought - a decent 12V to 30V converter would probably do and simplify everything. Not thought of that!I can't be sure though that it was snow on the panel, though it seems the most likely cause. We had vandals visiting too so it may be they fiddled with the settings on the MPPT charger or even broke the panel - that's tomorrow job.Oh and on my original query - still reading about batteries - I now found a bad idea to leave a LiFePO battery sat at 100% for ages anyway so a bit of a non starter.

author: Poroit
author_rank: Frequent Contributor
timestamp: January 28, 2025, 07:45:44 am
content: G'day jmh,This article may assist you.https://www.solarpowerworldonline.com/2019/03/connect-solar-controllers-parallel-meet-high-power-charging-requirements/Individual panels and controllers for each battery?My company always got good engineering support from a one stop shop company that supplied the PV, Batteries & Solar Chargers as they see more Project Applications.https://www.master-instruments.com.au/pages/perth-wa-branch.htmlThere is a Pommie guy there called Andy who may assist someone in the Old Country. He should remember what snow looked likeMaybe you have similar over there.

author: jmh
author_rank: Contributor
timestamp: January 28, 2025, 03:46:08 pm
content: Ok I had time today to bring my thinking brain along on the long walk. You need to bear in mind that heritage railways have very little money so anything not safety critical tends to be done piecemeal. Sorry this is long but may be of some interest...We have a solar panel which seems to be working fine as it gives around 29V off load in daylight and drops when we cover it up.We have a solar charger that can do 36V or 48V and expects a 24-ish V input for 36V output.And we have three 12V 110Ah flooded batteries.Now the important bit. Because the solar charger expects around 24V for a 36V output and the solar panel can float to around 40V in bright sunshine we have a 36/48V to 24V converter in between. When the panel was installed it was sunny and connecting the panel directly caused the solar charger to suck its thumb and ignore the input so the voltage converter was acquired.Piecemeal... all different suppliers rather than a kit as a solution. But needs must.When we arrived today the solar charger was not charging but was happy with the batteries that someone (not our team) had replaced since last week. It's worth saying here that we never get new batteries, any batteries we get will have seen service under a railway carriage for a while. The output from the voltage converter was well down, <10V, so the charger ignores it. The solar panel was managing 29V off load but only 16V or so when connected to the voltage converter, so well below spec for it. I've tested that before at home and the output drops rapidly once given input that is less than spec (I've lost my notes from last time so I forgot the voltage this happens at)So we've probably seen a drop in output from the solar panel so low that the solar charger gives up which is fair enough and it simply never got bright enough to recover, the batteries then self discharging over time. As it is the output from the panel is currently well within the range that the solar charger can handle so we just removed the converter and it is now happily charging. And will probably continue to do so until late spring when we actually get sun.Anyway, the suggestion of just using 12V and up converting to 36V is what we will try next given that it can be fully tested without actually removing the current kit. But as an interim solution, getting a voltage converter that will give a constant output over a wide range of input voltages seems a useful experiment. So I have (more) homework...

author: Peabody
author_rank: Super Contributor
timestamp: January 28, 2025, 05:37:34 pm
content: I'm just a hobbyist, and I have trouble understanding exactly how a buck/boost voltage converter would work with a solar panel. The problem is that the panel is basically a current generating device, with wildly fluctuating output voltage depending on the current draw. Well, perhaps this has been solved, but I don't see how a switching converter would work.I wonder if a simpler and less expensive solution might be to bypass the existing voltage converter when sunlight gets to be below a certain brightness. I don't think the switching can be based on panel output voltage because it will oscillate, but some kind of simple light sensor controlling a relay, with some hysteresis to prevent "flicker", might be good enough. You just need the converter to operate when it's too bright outside, but be bypassed otherwise. Even on a bright day, it would be automatically switched out when the sun is low in the sky.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 28, 2025, 07:38:40 pm
content: The problem is this extra "converter". Solar charge controller is really a specialized product, which does a specific task - even if it's cheap. Generic voltage converter modules will be unsuitable and just work in some lucky conditions, but "latch off" otherwise.You need to match the panel voltage to the charge controller. Charge controllers are usually specified with "nominal" panel voltage range, making this matching easy, but it's always good to double check that the charge controller maximum input voltage spec is at least 20-30% more than the "Open-Circuit Voltage" spec of the panel.

author: Marco
author_rank: Super Contributor
timestamp: January 28, 2025, 09:32:10 pm
content: Quote from: Peabody on January 28, 2025, 05:37:34 pmIt fills a capacitor and the voltage of the capacitor gets boosted.But that wasn't really the point here, the charger was charging a series stack of batteries, which means it has faster balance problems.So the proposed solution was for the charger to charge parallel batteries and boost it for the load

author: Abdul09
author_rank: Newbie
timestamp: February 01, 2025, 10:22:31 am
content: Well, batteries life is largely dependent on the depth of discharge and the conditions in which they are operating. As far as, weather is concerned, whenever there is prolonged snow; the batteries will not be charged to maximum level. Once batteries drop too low its difficult for the MPPT controller to fully reinstate them. Therefore, what you can do is set a low voltage limit or only discharge batteries to 50%. It can enhance life of batteries and on the other hand you can increase panel in parallel to increase current. As there is snow, there is less current available to charge batteries. Charge controller has only to supply that much of current or power what's available to it. Increase panel and set a low voltage limit for batteries.

author: jmh
author_rank: Contributor
timestamp: February 01, 2025, 11:52:08 am
content: Quote from: Abdul09 on February 01, 2025, 10:22:31 amIndeed - but this is self discharge as the batteries would not have been used at all for around 3 months. Operationally, discharge is dictated by the need to work the points. It is made more awkward due to two batteries falling to 9v and the charger then decides there is a fault and will not even try to charge so it never recovers. The existing charger expects 24v solar input and boosts to 36v which I always found an odd way to do it but that's what we had.We have a 12v solar charger that says it can take up to 75v solar input and now a 12-36v converter is on order to try. Actually, making it 12v has another benefit because we can charge a spare battery under the signal box which already has 12v solar power (note that the distance between there and the points is far too far to consider a cable!)

author: Abdul09
author_rank: Newbie
timestamp: February 02, 2025, 06:51:04 am
content: Yes this would be be a correct way to proceed and you would have installed 36V in first place.1. Move to 36V Solar Charger2. Increase PV Panel on the input side by keeping in view of new 36V charger Input specifications3. If Machine takes 7A at 30V if it's correct, You will require minimum 350Watts Solar on input side.4. Identify and Replace the dead battery after checking the Health, because it will weaken others in series.

author: Zucca
author_rank: Unknown
timestamp: February 12, 2025, 03:31:44 pm
content: Quote from: jmh on January 27, 2025, 02:49:34 pmCharge them to max 3.4V/cell and done.

author: jmh
author_rank: Contributor
timestamp: February 20, 2025, 11:56:58 am
content: Ok... so we got a 12v-36v converter as a test (rated at 36v 15a which means at least 45a on the 12v side but the leads look to be good for 5a max!!). It draws around 150mA off load and falls flat when asked to deliver current. I expected as much when I saw the thing. Although running it all off 12v would simplify things I overlooked the issue of how to either have very little continual drain (less than the solar input) when the output is not needed vs only connecting it to the 12v when it is. Energising a contractor rated at high current would be very tricky where things are. (yes I also suggested we just get a generator and run it when needed...)While fiddling I also contacted a local company that supply the Victron products. Victron have a 36v solar charger that claims it can cope with flat(-ish) batteries unlike the current setup. There is also a reporting module which uses cellular to send to an app, 5 years free license. I was advised to use a panel with at least 20v more than the expected load, so 60v (say) at least. Funny I never thought of that as it seems obvious. Ours does 42v. The company have not yet priced it all for me but it is affordable and having the reporting mechanism will help a lot. We can make use of the existing solar panel elsewhere for signal lighting.