daza avatar image
daza asked

Battery charger?

What are you guys using to charge a new 48volt battery? As it doesn’t look like I will have enough money to buy all the batteries I want in one. So I’m after a what solution that you use and to see if I can build it on my wall as a perm fixture so I’m not hunting in I need it or a battery goes wonky I could just disconnect and charge direct.

48volt charging
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matt1309 avatar image matt1309 commented ·

Hi @Daza

Not sure this is the answer you're after but this is what i do when i add new batteries. (on top of the standard stuff ie making sure the leads/resistance to each battery is the same)

My system is non critical so I can temporarily turn things off so I just use the main system to charge up new batteries (this may not be an option if yours is critical). When I added batteries I add the new batteries on a their own separate DC disconnect (and t class fuse).

ie Main disconnect then Battery 1 Disconnect and Battery 2 disconnect. (Also allows me to work on individual a battery without turning system off assuming 1 battery can handle all the load)

I'm also cheap so didn't want to buy another battery charger (the DIY batteries I built were 15kw so would take ages to charge with a cheaper charger)

Tbh I've seen folk just plug in new batteries (especially if they have a solid BMS/with current limiting built in) but I'm too nervous of inrush/balancing current due to varying initial voltages/SoC so i go through the below process to minimise risk of that.

I've seen @off-grid_garage on youtube test this by having one DIY LiFePO4 battery at low SoC and another at high and monitoring the inrush/balance of current when both are turned/connected in parallel (with individual BMS between them). However I'm too nervous to try this so i go through the below steps just in case.

1. Discharge current battery to around 50%.

2. Disconnect loads, then disconnect my current battery and connect new battery. via their individual disconnects

3. Use multiplus to fully charge the new battery, I do this with reduced charge current especially towards the end of full charge to allow BMS to balance cells slowly. (as i mentioned i built my own battery from cells so initial top balance can take awhile if cells are at very different SoCs so I'm much more comfortable if the charge current is reduced).

4. I then discharge new battery down to 50% per smart shunt and turn the old battery back on and adjust smart shunt. This should reduce inrush current (especially with LiFePO4 as voltages are relatively flat anyway so if i've lined up SoC close it should be even less) I find with this method the batteries are almost balanced when they're both turned on. However i do let batteries full charge once more to ensure they are.

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daza answered ·

Thanks @matt1309 ive watched a bit of off grid garage. kind of looking for a solution to fully charge it and straight forward, I’ve built a pre charge circuit so I won’t get the inrush as the capacitors would be precharged before the batteries are reconnected to the inverter.

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matt1309 avatar image matt1309 commented ·
Sorry by inrush current I meant from battery 1 to battery 2 if voltages were different and not from batteries to inverter capacitors.

But makes sense, no worries. I didn't buy a battery charger to charge it first as I knew I'd only use it once and the affordable ones had such low output I knew it would've taken ages to charge.

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daza avatar image daza matt1309 commented ·
Ah the differential in the batteries. Now you put it like only ever going to use it a couple of times I may as well get a small portable one as I would really need it to charge as a matter of urgency.
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