question

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Curtis asked

Lynx Distributor for parallel wired smart batteries (so no Lynx shunt or BMS) no to 2nd Lynx for DC loads

I’m building a 24v overlanding system with 3x 24v 200AH ‘smart’ batteries. These batteries have internal BMS and VE.can output so no shunt or external BMS is required, they plug straight into Cerbo.

I know this is getting to the extremes for a 24v system, but the battery configuration and available space (plus my PV panels) preclude going to 48v unless I drop capacity by 1/3. The batteries have a constant power rating of 150A and a peak rating of 300a each. Expected current draw is about 300 amps (2xMultis @~100a each, plus ~80a DC-DC) but max current for each Multi is ~250 and DC-DC is ~100a, taking me outside 4/0 cable rating. My plan is to limit this using Mega fuses to each Load and to each Battery.

I’m planning on using a Lynx Distributor for connecting the batteries in parallel and then another Lynx Distributor to connect my loads (2x Multiplus 24/3000/70 in split phase, 150/85 MPPT & DC-DC 24/12) and allow me to fuse everything individually.

Wth no Lynx Shunt or Lynx BMS required, can I connect the two Lynx Distributors together and, if so, would this not be counter to the Victron (and other) wiring suggestions where you should draw the positive from the nearest battery and the negative from the furthest battery in a parallel configuration?

Should I use 4/0 cables to take the negative from the right hand (ie ‘furthest’) side of the ‘Battery’ Distributor negative bus bar to the right side busbar of the ‘Load‘ Distributor, andbthen using an equal length cable connect the left hand (‘near’) positive busbar of the Battery Distributor to the right side positive busbar of the Load Distributor, instead of connecting the 2 Distributors together?

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1 Answer
Kevin Windrem avatar image
Kevin Windrem answered ·

If practical, route each battery to the Lynx and fuse separately. That way you can use smaller wires and you get a better balance between batteries as well. Match wire lengths to each battery (i.e same length for positive connection, same length for negative connection, positive and negative can be different length). Wire size should be calculated both for ampacity (continuous current for each component (battery or Multi) and for voltage drop at peak current.

The trick to take power from opposite corners of a battery bank works fine for two batteries but doesn't fully balance load and charging current with three or more batteries in parallel. A bus bar (Lynx) is highly recommended for more than two batteries.

Do not rely on fused or circuit breakers to limit current draw in the system. This would almost guarantee the fuse would blow or circuit breaker would trip in a fully loaded system.

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Curtis avatar image Curtis commented ·

Thanks Kevin, so if I connect each battery to one Lynx Distributor (using 2/0 cables of equal length) I should then connect that ‘battery’ Lynx Distributor directly to my ‘load’ Lynx Distributor (via the Lynx bus bars) and connect each Multi, MPPT and DC-DC bus to it, using appropriate wiring and fuses for the constant current load for each load or charge source, yes? In this case, is there any issue with using the 4th (empty) input on my ‘battery’ Lynx Distributor for another load, such as an alternator charge input?


Thanks kindly for your time and advice here.

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Kevin Windrem avatar image Kevin Windrem Curtis commented ·
No, you need to separate the The input Lynx from the output Line and have some kind of disconnect for the system. You should also have a "precharge" circuit to avoid huge inrush current when turning on the disconnect. REC makes one if you don't want to spend $$$$ on a Lynx BMS which includes a disconnect (and a shunt).
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Curtis avatar image Curtis Kevin Windrem commented ·

The batteries I’m using (https://volthium.com/wp-content/uploads/2022/08/2022-Technical-Sheet-25V200A-1.pdf) have integrated BMS and VE.can communications. The manufacturer says that no shunt or external BMS is required as it will communicate directly with Victron GX (in my case, a Cerbo) but they have not mentioned specifically any pre charge circuit functionality.

I was planning on individual battery disconnects between the ‘battery’ Lynx and each battery (as well as between each charge/load source and my ‘loads’ Lynx too) but you are suggesting a master battery disconnect between my ‘load’ Lynx Distributor and my ‘battery‘ Lynx distributor. Would this would have to be rated for the total peak amps of the combined battery bank, ie 900A @ 24vdc?


Thanks again for your time & assistance.

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Kevin Windrem avatar image Kevin Windrem Curtis commented ·

A disconnect would need to be rated for the maximum system current which is hopefully less than the peak battery current.

Precharge:

When the inverter is connected to the DC bus there is a HUGE inrush current because of the capacitors across it's DC input. This current can trip the battery BMS and/or degrade the disconnect switch contacts due to arcing. A precharge circuit eliminates the inrush by charging the capacitors through a resistor so they charge slowly. The precharge circuit can simply be a resistor you manually switch across the DC system switch or could be automated. In an automated precharge, the power switch starts the process by closing a small relay that places the resistor across the main contacts. Then after a preset amount of time or when the voltage across the main contacts is near zero, the circuit then closes the main contactor. The Lynx BMS includes an automated precharge but is an expensive solution especially if you don't need a Victron BMS and shunt. Rec-Bms also sells a precharge system and contactor:

https://www.rec-bms.com/rec-accessories/

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